diff --git a/source/Provider.tsx b/source/Provider.tsx
index 613dce56a..919167db2 100644
--- a/source/Provider.tsx
+++ b/source/Provider.tsx
@@ -95,9 +95,10 @@ export default class Provider extends PureComponent<ProviderProps> {
 		this.props.tracker.disconnectFromHistory()
 	}
 	render() {
-		const iframeCouleur = new URLSearchParams(
-			document?.location.search.substring(1)
-		).get('couleur')
+		const iframeCouleur =
+			new URLSearchParams(document?.location.search.substring(1)).get(
+				'couleur'
+			) ?? undefined
 		return (
 			// If IE < 11 display nothing
 			<ReduxProvider store={this.store}>
diff --git a/source/Tracker.ts b/source/Tracker.ts
index dcd80230f..830ee274b 100644
--- a/source/Tracker.ts
+++ b/source/Tracker.ts
@@ -12,8 +12,8 @@ type PushType = (args: PushArgs) => void
 
 export default class Tracker {
 	push: PushType
-	unlistenFromHistory: () => void
-	previousPath: string
+	unlistenFromHistory: (() => void) | undefined
+	previousPath: string | undefined
 
 	constructor(pushFunction: PushType = args => window._paq.push(args)) {
 		if (typeof window !== 'undefined') window._paq = window._paq || []
diff --git a/source/actions/actions.ts b/source/actions/actions.ts
index 33c81d739..72800aa11 100644
--- a/source/actions/actions.ts
+++ b/source/actions/actions.ts
@@ -43,12 +43,17 @@ type DeletePreviousSimulationAction = {
 	type: 'DELETE_PREVIOUS_SIMULATION'
 }
 
-type SetExempleAction = {
-	type: 'SET_EXAMPLE'
-	name: null | string
-	situation?: object
-	dottedName?: string
-}
+type SetExempleAction =
+	| {
+			type: 'SET_EXAMPLE'
+			name: null
+	  }
+	| {
+			type: 'SET_EXAMPLE'
+			name: string
+			situation: object
+			dottedName: DottedName
+	  }
 
 type ResetSimulationAction = ReturnType<typeof resetSimulation>
 type UpdateAction = ReturnType<typeof updateSituation>
@@ -142,7 +147,8 @@ export const goBackToSimulation = (): ThunkResult<void> => (
 	{ history }
 ) => {
 	dispatch({ type: 'SET_EXAMPLE', name: null })
-	history.push(getState().simulation.url)
+	const url = getState().simulation?.url
+	url && history.push(url)
 }
 
 export function loadPreviousSimulation() {
@@ -155,7 +161,7 @@ export function hideControl(id: string) {
 	return { type: 'HIDE_CONTROL', id } as const
 }
 
-export const explainVariable = (variableName = null) =>
+export const explainVariable = (variableName: DottedName | null = null) =>
 	({
 		type: 'EXPLAIN_VARIABLE',
 		variableName
diff --git a/source/api/sirene.ts b/source/api/sirene.ts
index 8df00bf7a..fb9fdd28a 100644
--- a/source/api/sirene.ts
+++ b/source/api/sirene.ts
@@ -30,7 +30,9 @@ export type Etablissement = {
 	denomination?: string
 }
 
-async function searchFullText(text: string): Promise<Array<Etablissement>> {
+async function searchFullText(
+	text: string
+): Promise<Array<Etablissement> | null> {
 	const response = await fetch(
 		`https://entreprise.data.gouv.fr/api/sirene/v1/full_text/${text}?per_page=5`
 	)
diff --git a/source/components/CurrencyInput/CurrencyInput.tsx b/source/components/CurrencyInput/CurrencyInput.tsx
index f8d892146..eb5dd5710 100644
--- a/source/components/CurrencyInput/CurrencyInput.tsx
+++ b/source/components/CurrencyInput/CurrencyInput.tsx
@@ -25,11 +25,11 @@ export default function CurrencyInput({
 	const [initialValue, setInitialValue] = useState(valueProp)
 	const [currentValue, setCurrentValue] = useState(valueProp)
 	const onChangeDebounced = useRef(
-		debounceTimeout ? debounce(debounceTimeout, onChange) : onChange
+		debounceTimeout && onChange ? debounce(debounceTimeout, onChange) : onChange
 	)
 	// We need some mutable reference because the <NumberFormat /> component doesn't provide
 	// the DOM `event` in its custom `onValueChange` handler
-	const nextValue = useRef(null)
+	const nextValue = useRef('')
 
 	const inputRef = useRef<HTMLInputElement>()
 
@@ -51,8 +51,8 @@ export default function CurrencyInput({
 			...event.target,
 			value: nextValue.current
 		}
-		nextValue.current = null
-		onChangeDebounced.current(event)
+		nextValue.current = ''
+		onChangeDebounced.current?.(event)
 	}
 
 	const {
@@ -71,7 +71,7 @@ export default function CurrencyInput({
 		<div
 			className={classnames(className, 'currencyInput__container')}
 			{...(valueLength > 5 ? { style: { width } } : {})}
-			onClick={() => inputRef.current.focus()}
+			onClick={() => inputRef.current?.focus()}
 		>
 			{!currentValue && isCurrencyPrefixed && currencySymbol}
 			<NumberFormat
diff --git a/source/components/Feedback/FeedbackForm.tsx b/source/components/Feedback/FeedbackForm.tsx
index 7dc28c9de..23859407f 100644
--- a/source/components/Feedback/FeedbackForm.tsx
+++ b/source/components/Feedback/FeedbackForm.tsx
@@ -6,7 +6,7 @@ import { Trans } from 'react-i18next'
 type Props = { onEnd: () => void; onCancel: () => void }
 
 export default function FeedbackForm({ onEnd, onCancel }: Props) {
-	const formRef = useRef<HTMLFormElement>()
+	const formRef = useRef<HTMLFormElement>(null)
 	const tracker = useContext(TrackerContext)
 
 	const handleFormSubmit = (e: React.FormEvent): void => {
@@ -14,7 +14,7 @@ export default function FeedbackForm({ onEnd, onCancel }: Props) {
 		e.preventDefault()
 		fetch('/', {
 			method: 'POST',
-			body: new FormData(formRef.current)
+			body: new FormData(formRef.current ?? undefined)
 		})
 		onEnd()
 	}
diff --git a/source/components/FindCompany.tsx b/source/components/FindCompany.tsx
index 351b073ce..71c007abe 100644
--- a/source/components/FindCompany.tsx
+++ b/source/components/FindCompany.tsx
@@ -7,7 +7,9 @@ import { Etablissement, searchDenominationOrSiren } from '../api/sirene'
 import { debounce } from '../utils'
 
 export default function Search() {
-	const [searchResults, setSearchResults] = useState<Array<Etablissement>>()
+	const [searchResults, setSearchResults] = useState<Array<
+		Etablissement
+	> | null>()
 	const [isLoading, setLoadingState] = useState(false)
 
 	const handleSearch = useCallback(
diff --git a/source/components/QuickLinks.tsx b/source/components/QuickLinks.tsx
index bbcee1971..d4886f2e6 100644
--- a/source/components/QuickLinks.tsx
+++ b/source/components/QuickLinks.tsx
@@ -30,24 +30,26 @@ export default function QuickLinks() {
 		toPairs
 	)(quickLinks)
 
+	if (links.length < 1) {
+		return null
+	}
+
 	return (
-		!!links.length && (
-			<span>
-				<small>Questions :</small>
-				{links.map(([label, dottedName]) => (
-					<button
-						key={dottedName}
-						className={`ui__ link-button ${
-							dottedName === currentQuestion ? 'active' : ''
-						}`}
-						css="margin: 0 0.4rem !important"
-						onClick={() => dispatch(goToQuestion(dottedName))}
-					>
-						<T k={'quicklinks.' + label}>{label}</T>
-					</button>
-				))}{' '}
-				{/* <button className="ui__ link-button">Voir la liste</button> */}
-			</span>
-		)
+		<span>
+			<small>Questions :</small>
+			{links.map(([label, dottedName]) => (
+				<button
+					key={dottedName}
+					className={`ui__ link-button ${
+						dottedName === currentQuestion ? 'active' : ''
+					}`}
+					css="margin: 0 0.4rem !important"
+					onClick={() => dispatch(goToQuestion(dottedName))}
+				>
+					<T k={'quicklinks.' + label}>{label}</T>
+				</button>
+			))}{' '}
+			{/* <button className="ui__ link-button">Voir la liste</button> */}
+		</span>
 	)
 }
diff --git a/source/components/SalaryExplanation.tsx b/source/components/SalaryExplanation.tsx
index e267652a0..add7f7459 100644
--- a/source/components/SalaryExplanation.tsx
+++ b/source/components/SalaryExplanation.tsx
@@ -34,7 +34,7 @@ export default function SalaryExplanation() {
 	const showDistributionFirst = useSelector(
 		(state: RootState) => !state.conversationSteps.foldedSteps.length
 	)
-	const distributionRef = useRef<HTMLDivElement>()
+	const distributionRef = useRef<HTMLDivElement>(null)
 	return (
 		<ErrorBoundary>
 			<Animate.fromTop key={showDistributionFirst.toString()}>
@@ -51,7 +51,7 @@ export default function SalaryExplanation() {
 							<button
 								className="ui__ small simple button"
 								onClick={() =>
-									distributionRef.current.scrollIntoView({
+									distributionRef.current?.scrollIntoView({
 										behavior: 'smooth',
 										block: 'start'
 									})
@@ -130,7 +130,7 @@ function PaySlipSection() {
 	return (
 		<section>
 			<h2>
-				{unit.endsWith('mois') ? (
+				{unit?.endsWith('mois') ? (
 					<Trans>Fiche de paie</Trans>
 				) : (
 					<Trans>Détail annuel des cotisations</Trans>
diff --git a/source/components/SearchBar.tsx b/source/components/SearchBar.tsx
index 908b512a6..bc71d08c2 100644
--- a/source/components/SearchBar.tsx
+++ b/source/components/SearchBar.tsx
@@ -17,6 +17,8 @@ type SearchBarProps = {
 	finally?: () => void
 }
 
+type Option = Pick<Rule, 'dottedName' | 'name' | 'title'>
+
 export default function SearchBar({
 	rules,
 	showDefaultList,
@@ -24,7 +26,7 @@ export default function SearchBar({
 }: SearchBarProps) {
 	const sitePaths = useContext(SitePathsContext)
 	const [input, setInput] = useState('')
-	const [selectedOption, setSelectedOption] = useState(null)
+	const [selectedOption, setSelectedOption] = useState<Option | null>(null)
 	const [results, setResults] = useState([])
 	const { i18n } = useTranslation()
 
@@ -44,7 +46,7 @@ export default function SearchBar({
 		return <ul>{options.map(option => renderOption(option))}</ul>
 	}
 
-	let renderOption = option => {
+	let renderOption = (option: Option) => {
 		let { title, dottedName, name } = option
 		return (
 			<li
@@ -86,14 +88,14 @@ export default function SearchBar({
 				>
 					<Highlighter
 						searchWords={[input]}
-						textToHighlight={title || capitalise0(name)}
+						textToHighlight={title || capitalise0(name) || ''}
 					/>
 				</Link>
 			</li>
 		)
 	}
 
-	if (selectedOption != null) {
+	if (selectedOption !== null) {
 		finallyCallback && finallyCallback()
 		return (
 			<Redirect
diff --git a/source/components/StackedBarChart.tsx b/source/components/StackedBarChart.tsx
index 66e013baa..a9578f480 100644
--- a/source/components/StackedBarChart.tsx
+++ b/source/components/StackedBarChart.tsx
@@ -87,7 +87,7 @@ export default function StackedBarChart({ data }: StackedBarChartProps) {
 	const [intersectionRef, displayChart] = useDisplayOnIntersecting({
 		threshold: 0.5
 	})
-	const percentages = roundedPercentages(data.map(d => d.nodeValue))
+	const percentages = roundedPercentages(data.map(d => d.nodeValue || 0))
 	const dataWithPercentage = data.map((data, index) => ({
 		...data,
 		percentage: percentages[index]
diff --git a/source/components/TargetSelection.tsx b/source/components/TargetSelection.tsx
index ce1b41008..55e26b7c9 100644
--- a/source/components/TargetSelection.tsx
+++ b/source/components/TargetSelection.tsx
@@ -274,7 +274,7 @@ let TargetInputOrValue = ({
 				</>
 			) : (
 				<span>
-					{Number.isNaN(value) ? '—' : formatCurrency(value, language)}
+					{value && Number.isNaN(value) ? '—' : formatCurrency(value, language)}
 				</span>
 			)}
 			{target.dottedName.includes('prix du travail') && <AidesGlimpse />}
diff --git a/source/components/Value.tsx b/source/components/Value.tsx
index c6be2761e..6dd858c61 100644
--- a/source/components/Value.tsx
+++ b/source/components/Value.tsx
@@ -47,7 +47,7 @@ export default function Value({
 
 	if (
 		(nilValueSymbol !== undefined && nodeValue === 0) ||
-		Number.isNaN(nodeValue) ||
+		(nodeValue && Number.isNaN(nodeValue)) ||
 		nodeValue === null
 	)
 		return (
@@ -63,7 +63,7 @@ export default function Value({
 				(nodeValue as any).nom
 			) : valueType === 'boolean' ? (
 				booleanTranslations[language][nodeValue]
-			) : (
+			) : nodeValue !== undefined ? (
 				formatValue({
 					minimumFractionDigits,
 					maximumFractionDigits,
@@ -71,7 +71,7 @@ export default function Value({
 					unit,
 					value: nodeValue
 				})
-			)
+			) : null
 	return nodeValue == undefined ? null : (
 		<span css={style(customCSS)} className="value">
 			{negative ? '-' : ''}
diff --git a/source/components/conversation/InputSuggestions.tsx b/source/components/conversation/InputSuggestions.tsx
index 34f253846..0f9a0f020 100644
--- a/source/components/conversation/InputSuggestions.tsx
+++ b/source/components/conversation/InputSuggestions.tsx
@@ -18,9 +18,9 @@ export default function InputSuggestions({
 	onFirstClick,
 	unit
 }: InputSuggestionsProps) {
-	const [suggestion, setSuggestion] = useState(null)
+	const [suggestion, setSuggestion] = useState<number>()
 	const { t } = useTranslation()
-	const defaultUnit = parseUnit(useSelector(defaultUnitsSelector)[0])
+	const defaultUnit = parseUnit(useSelector(defaultUnitsSelector)[0] ?? '')
 	if (!suggestions) return null
 
 	return (
diff --git a/source/components/ui/AnimatedTargetValue.tsx b/source/components/ui/AnimatedTargetValue.tsx
index 020c574a5..2d86bc464 100644
--- a/source/components/ui/AnimatedTargetValue.tsx
+++ b/source/components/ui/AnimatedTargetValue.tsx
@@ -11,7 +11,7 @@ type AnimatedTargetValueProps = {
 	children?: React.ReactNode
 }
 
-const formatDifference: typeof formatCurrency = (difference, language) => {
+const formatDifference = (difference: number, language: string) => {
 	const prefix = difference > 0 ? '+' : ''
 	return prefix + formatCurrency(difference, language)
 }
@@ -25,12 +25,12 @@ export default function AnimatedTargetValue({
 
 	// We don't want to show the animated if the difference comes from a change in the unit
 	const currentUnit = useSelector(
-		(state: RootState) => state.simulation.defaultUnits[0]
+		(state: RootState) => state?.simulation?.defaultUnits[0]
 	)
 	const previousUnit = useRef(currentUnit)
 
 	const difference =
-		previousValue.current === value || Number.isNaN(value)
+		previousValue.current === value || (value && Number.isNaN(value))
 			? null
 			: (value || 0) - (previousValue.current || 0)
 	const shouldDisplayDifference =
@@ -44,7 +44,7 @@ export default function AnimatedTargetValue({
 	return (
 		<>
 			<span className="Rule-value">
-				{shouldDisplayDifference && (
+				{shouldDisplayDifference && difference !== null && (
 					<Evaporate
 						style={{
 							color: difference > 0 ? 'chartreuse' : 'red',
diff --git a/source/components/utils/persistState.ts b/source/components/utils/persistState.ts
index 084c2c7c0..014c9df9f 100644
--- a/source/components/utils/persistState.ts
+++ b/source/components/utils/persistState.ts
@@ -15,7 +15,7 @@ export const getInitialState = (key: string) => {
 		return
 	}
 	try {
-		return JSON.parse(safeLocalStorage.getItem(key))
+		return JSON.parse(value)
 	} catch (e) {
 		console.warn(e)
 		return null
diff --git a/source/components/utils/useDisplayOnIntersecting.ts b/source/components/utils/useDisplayOnIntersecting.ts
index 26096ee88..4709a594a 100644
--- a/source/components/utils/useDisplayOnIntersecting.ts
+++ b/source/components/utils/useDisplayOnIntersecting.ts
@@ -5,7 +5,7 @@ export default function({
 	rootMargin,
 	threshold = 0
 }: IntersectionObserverInit): [React.RefObject<HTMLDivElement>, boolean] {
-	const ref = useRef<HTMLDivElement>()
+	const ref = useRef<HTMLDivElement>(null)
 	const [wasOnScreen, setWasOnScreen] = useState(false)
 
 	useEffect(() => {
@@ -13,7 +13,7 @@ export default function({
 			([entry]) => {
 				if (entry.isIntersecting) {
 					setWasOnScreen(entry.isIntersecting)
-					observer.unobserve(ref.current)
+					ref.current && observer.unobserve(ref.current)
 				}
 			},
 			{
@@ -23,11 +23,11 @@ export default function({
 			}
 		)
 		const node = ref.current
-		if (ref.current) {
+		if (node) {
 			observer.observe(node)
 		}
 		return () => {
-			observer.unobserve(node)
+			node && observer.unobserve(node)
 		}
 	}, [root, rootMargin, threshold])
 
diff --git a/source/components/utils/withColours.tsx b/source/components/utils/withColours.tsx
index fc93f7496..3621f15ed 100644
--- a/source/components/utils/withColours.tsx
+++ b/source/components/utils/withColours.tsx
@@ -97,7 +97,7 @@ export type ThemeColours = ReturnType<typeof generateTheme>
 export const ThemeColoursContext = createContext<ThemeColours>(generateTheme())
 
 type ProviderProps = {
-	colour: string
+	colour?: string
 	children: React.ReactNode
 }
 
diff --git a/source/engine/format.ts b/source/engine/format.ts
index 84e79f32d..dbb59df43 100644
--- a/source/engine/format.ts
+++ b/source/engine/format.ts
@@ -35,7 +35,7 @@ export let numberFormatter = ({
 	}).format(value)
 }
 
-export const currencyFormat = (language: string) => ({
+export const currencyFormat = (language: string | undefined) => ({
 	isCurrencyPrefixed: !!numberFormatter({ language, style: 'currency' })(
 		12
 	).match(/^€/),
@@ -43,13 +43,13 @@ export const currencyFormat = (language: string) => ({
 	decimalSeparator: numberFormatter({ language })(0.1).charAt(1)
 })
 
-export const formatCurrency = (value: number, language: string) => {
+export const formatCurrency = (value: number | undefined, language: string) => {
 	return value == null
 		? ''
 		: formatValue({ unit: '€', language, value }).replace(/^(-)?€/, '$1€\u00A0')
 }
 
-export const formatPercentage = value =>
+export const formatPercentage = (value: number | undefined) =>
 	value == null
 		? ''
 		: formatValue({ unit: '%', value, maximumFractionDigits: 2 })
@@ -58,7 +58,7 @@ export type formatValueOptions = {
 	maximumFractionDigits?: number
 	minimumFractionDigits?: number
 	language?: string
-	unit: Unit | string
+	unit?: Unit | string
 	value: number
 }
 
@@ -72,7 +72,7 @@ export function formatValue({
 	if (typeof value !== 'number') {
 		return value
 	}
-	const serializedUnit = serialiseUnit(unit, value, language)
+	const serializedUnit = unit ? serialiseUnit(unit, value, language) : undefined
 
 	switch (serializedUnit) {
 		case '€':
diff --git a/source/engine/generateQuestions.ts b/source/engine/generateQuestions.ts
index 0168a8695..4a7338ce3 100644
--- a/source/engine/generateQuestions.ts
+++ b/source/engine/generateQuestions.ts
@@ -15,6 +15,7 @@ import {
 	toPairs,
 	values
 } from 'ramda'
+import { DottedName } from 'Types/rule'
 
 /*
 	COLLECTE DES VARIABLES MANQUANTES
@@ -31,7 +32,12 @@ import {
 	missingVariables: {variable: [objectives]}
  */
 
-export let collectMissingVariablesByTarget = (targets = []) =>
+type Explanation = {
+	missingVariables: Array<DottedName>
+	dottedName: DottedName
+}
+
+export let collectMissingVariablesByTarget = (targets: Explanation[] = []) =>
 	fromPairs(targets.map(target => [target.dottedName, target.missingVariables]))
 
 export let getNextSteps = missingVariablesByTarget => {
diff --git a/source/engine/mecanisms/encadrement.tsx b/source/engine/mecanisms/encadrement.tsx
index 341749a0c..6a8ec6bd1 100644
--- a/source/engine/mecanisms/encadrement.tsx
+++ b/source/engine/mecanisms/encadrement.tsx
@@ -24,8 +24,9 @@ function MecanismEncadrement({ nodeValue, explanation, unit }) {
 						{!explanation.plancher.isDefault && (
 							<span
 								css={
-									nodeValue === val(explanation.plancher) &&
-									'background: yellow'
+									nodeValue === val(explanation.plancher)
+										? 'background: yellow'
+										: {}
 								}
 							>
 								<strong className="key">Minimum : </strong>
@@ -35,7 +36,9 @@ function MecanismEncadrement({ nodeValue, explanation, unit }) {
 						{!explanation.plafond.isDefault && (
 							<span
 								css={
-									nodeValue === val(explanation.plafond) && 'background: yellow'
+									nodeValue === val(explanation.plafond)
+										? 'background: yellow'
+										: {}
 								}
 							>
 								<strong className="key">Plafonné à : </strong>
diff --git a/source/engine/traverse.ts b/source/engine/traverse.ts
index 0ac4aa34c..d0d7c3eac 100644
--- a/source/engine/traverse.ts
+++ b/source/engine/traverse.ts
@@ -131,14 +131,16 @@ type CacheMeta = {
 	}
 }
 
-export let analyseMany = (parsedRules, targetNames, defaultUnits = []) => (
-	situationGate: (name: DottedName) => any
-) => {
+export let analyseMany = (
+	parsedRules,
+	targetNames,
+	defaultUnits: Array<string> = []
+) => (situationGate: (name: DottedName) => any) => {
 	// TODO: we should really make use of namespaces at this level, in particular
 	// setRule in Rule.js needs to get smarter and pass dottedName
-	defaultUnits = defaultUnits.map(parseUnit)
+	const defaultParsedUnits = defaultUnits.map(parseUnit)
 	let cache = {
-		_meta: { contextRule: [], defaultUnits } as CacheMeta
+		_meta: { contextRule: [], defaultUnits: defaultParsedUnits } as CacheMeta
 	}
 
 	let parsedTargets = targetNames.map(t => {
diff --git a/source/engine/units.ts b/source/engine/units.ts
index 8b9f01e1f..16fd87048 100644
--- a/source/engine/units.ts
+++ b/source/engine/units.ts
@@ -86,7 +86,7 @@ let noUnit = { numerators: [], denominators: [] }
 export let inferUnit = (
 	operator: SupportedOperators,
 	rawUnits: Array<Unit>
-): Unit => {
+): Unit | undefined => {
 	let units = rawUnits.map(u => u || noUnit)
 	if (operator === '*')
 		return simplify({
@@ -109,7 +109,7 @@ export let inferUnit = (
 		return rawUnits.find(u => u)
 	}
 
-	return null
+	return undefined
 }
 
 export let removeOnce = <T>(
diff --git a/source/reducers/inFranceAppReducer.ts b/source/reducers/inFranceAppReducer.ts
index de30645a7..82254e9ab 100644
--- a/source/reducers/inFranceAppReducer.ts
+++ b/source/reducers/inFranceAppReducer.ts
@@ -78,10 +78,7 @@ function companyCreationChecklist(
 	}
 }
 
-function companyStatusChoice(
-	state: LegalStatus = null,
-	action: Action
-): LegalStatus {
+function companyStatusChoice(state: LegalStatus | null = null, action: Action) {
 	if (action.type === 'RESET_COMPANY_STATUS_CHOICE') {
 		return null
 	}
@@ -131,7 +128,7 @@ export type Company = {
 	siren: string
 	catégorieJuridique?: string
 	statutJuridique?: string
-	dateDébutActivité?: Date
+	dateDébutActivité?: string
 	isAutoEntrepreneur?: boolean
 	isDirigeantMajoritaire?: boolean
 	localisation?: GeoDetails & {
@@ -140,7 +137,7 @@ export type Company = {
 	}
 }
 
-function existingCompany(state: Company = null, action): Company {
+function existingCompany(state: Company | null = null, action): Company | null {
 	if (!action.type.startsWith('EXISTING_COMPANY::')) {
 		return state
 	}
diff --git a/source/reducers/rootReducer.ts b/source/reducers/rootReducer.ts
index bd6003b65..e0de4f39a 100644
--- a/source/reducers/rootReducer.ts
+++ b/source/reducers/rootReducer.ts
@@ -1,6 +1,6 @@
 import { Action } from 'Actions/actions'
 import { Analysis } from 'Engine/traverse'
-import { areUnitConvertible, convertUnit, parseUnit } from 'Engine/units'
+import { areUnitConvertible, convertUnit, parseUnit, Unit } from 'Engine/units'
 import {
 	defaultTo,
 	dissoc,
@@ -22,10 +22,7 @@ import i18n, { AvailableLangs } from '../i18n'
 import inFranceAppReducer from './inFranceAppReducer'
 import storageRootReducer from './storageReducer'
 
-function explainedVariable(
-	state: DottedName = null,
-	action: Action
-): DottedName {
+function explainedVariable(state: DottedName | null = null, action: Action) {
 	switch (action.type) {
 		case 'EXPLAIN_VARIABLE':
 			return action.variableName
@@ -36,17 +33,23 @@ function explainedVariable(
 	}
 }
 
-function currentExample(state = null, action: Action) {
+type Example = null | {
+	name: string
+	situation: object
+	dottedName: DottedName
+	defaultUnits?: Array<Unit>
+}
+
+function currentExample(state: Example = null, action: Action): Example {
 	switch (action.type) {
 		case 'SET_EXAMPLE':
-			const { situation, name, dottedName } = action
-			return name != null ? { name, situation, dottedName } : null
+			return action.name != null ? action : null
 		default:
 			return state
 	}
 }
 
-function situationBranch(state: number = null, action: Action): number {
+function situationBranch(state: number | null = null, action: Action) {
 	switch (action.type) {
 		case 'SET_SITUATION_BRANCH':
 			return action.id
@@ -55,10 +58,7 @@ function situationBranch(state: number = null, action: Action): number {
 	}
 }
 
-function activeTargetInput(
-	state: DottedName | null = null,
-	action: Action
-): DottedName | null {
+function activeTargetInput(state: DottedName | null = null, action: Action) {
 	switch (action.type) {
 		case 'SET_ACTIVE_TARGET_INPUT':
 			return action.name
@@ -83,7 +83,7 @@ function lang(
 
 type ConversationSteps = {
 	foldedSteps: Array<DottedName>
-	unfoldedStep?: DottedName
+	unfoldedStep?: DottedName | null
 }
 
 function conversationSteps(
@@ -178,9 +178,9 @@ export type Simulation = {
 }
 
 function simulation(
-	state: Simulation = null,
+	state: Simulation | null = null,
 	action: Action,
-	analysis: Analysis | Array<Analysis>
+	analysis: Analysis | Array<Analysis> | null
 ): Simulation | null {
 	if (action.type === 'SET_SIMULATION') {
 		const { config, url } = action
@@ -272,9 +272,9 @@ const mainReducer = (state, action: Action) =>
 		rules: defaultTo(null) as Reducer<Array<Rule>>,
 		explainedVariable,
 		// We need to access the `rules` in the simulation reducer
-		simulation: (a: Simulation | null, b: Action): Simulation =>
+		simulation: (a: Simulation | null = null, b: Action): Simulation | null =>
 			simulation(a, b, a && analysisWithDefaultsSelector(state)),
-		previousSimulation: defaultTo(null) as Reducer<SavedSimulation>,
+		previousSimulation: defaultTo(null) as Reducer<SavedSimulation | null>,
 		currentExample,
 		situationBranch,
 		activeTargetInput,
@@ -285,6 +285,6 @@ export default reduceReducers<RootState>(
 	existingCompanyRootReducer,
 	mainReducer as any,
 	storageRootReducer as any
-)
+) as Reducer<RootState>
 
 export type RootState = ReturnType<typeof mainReducer>
diff --git a/source/selectors/analyseSelectors.ts b/source/selectors/analyseSelectors.ts
index 490a8dfe0..7e574c6f2 100644
--- a/source/selectors/analyseSelectors.ts
+++ b/source/selectors/analyseSelectors.ts
@@ -27,7 +27,7 @@ import {
 	zipWith
 } from 'ramda'
 import { useSelector } from 'react-redux'
-import { RootState } from 'Reducers/rootReducer'
+import { RootState, Simulation } from 'Reducers/rootReducer'
 import { createSelector, createSelectorCreator, defaultMemoize } from 'reselect'
 import { DottedName } from 'Types/rule'
 import { mapOrApply } from '../utils'
@@ -51,7 +51,7 @@ export let ruleDefaultsSelector = createSelector([flatRulesSelector], rules =>
 export let targetNamesSelector = (state: RootState) => {
 	let objectifs = configSelector(state).objectifs
 	if (!objectifs || !Array.isArray(objectifs)) {
-		return null
+		return []
 	}
 	const targetNames = [].concat(
 		...(objectifs as any).map(objectifOrGroup =>
@@ -121,9 +121,7 @@ const createSituationBrancheSelector = (
 			situation,
 			branches,
 			configSituation
-		):
-			| RootState['simulation']['situation']
-			| Array<RootState['simulation']['situation']> => {
+		): Simulation['situation'] | Array<Simulation['situation']> => {
 			if (branches) {
 				return branches.map(({ situation: branchSituation }) => ({
 					...configSituation,
@@ -215,7 +213,7 @@ export let exampleAnalysisSelector = createSelector(
 			rules,
 			dottedName,
 			(dottedName: DottedName) => situation[dottedName],
-			example.defaultUnits
+			example?.defaultUnits
 		)
 )
 
diff --git a/source/selectors/companyStatusSelectors.ts b/source/selectors/companyStatusSelectors.ts
index f970c84bd..492f51128 100644
--- a/source/selectors/companyStatusSelectors.ts
+++ b/source/selectors/companyStatusSelectors.ts
@@ -12,6 +12,7 @@ import {
 	mergeWith,
 	sortBy
 } from 'ramda'
+import { RootState } from 'Reducers/rootReducer'
 import { LegalStatusRequirements, State } from 'Types/companyTypes'
 
 const LEGAL_STATUS_DETAILS = {
@@ -125,9 +126,7 @@ export const possibleStatusSelector = (state: {
 }): Record<LegalStatus, boolean> =>
 	possibleStatus(state.inFranceApp.companyLegalStatus)
 
-export const nextQuestionSelector = (state: {
-	inFranceApp: State
-}): Question => {
+export const nextQuestionSelector = (state: RootState): Question | null => {
 	const legalStatusRequirements = state.inFranceApp.companyLegalStatus
 	const questionAnswered = Object.keys(legalStatusRequirements) as Array<
 		Question
@@ -167,7 +166,7 @@ export const nextQuestionSelector = (state: {
 }
 
 export const nextQuestionUrlSelector = (
-	state: { inFranceApp: State },
+	state: RootState,
 	{ sitePaths }: { sitePaths: SitePaths }
 ) => {
 	const nextQuestion = nextQuestionSelector(state)
diff --git a/source/selectors/ficheDePaieSelectors.ts b/source/selectors/ficheDePaieSelectors.ts
index 5617e0a81..f5c06dd6a 100644
--- a/source/selectors/ficheDePaieSelectors.ts
+++ b/source/selectors/ficheDePaieSelectors.ts
@@ -98,7 +98,7 @@ export let analysisToCotisations = (analysis: Analysis) => {
 	]
 		.map(name => analysis.cache[name])
 		.map(pathOr([], ['explanation', 'formule', 'explanation', 'explanation']))
-		.reduce(concat, [])
+		.reduce(concat as any, [])
 
 	const cotisations = pipe(
 		map((rule: any) =>
diff --git a/source/selectors/storageSelectors.ts b/source/selectors/storageSelectors.ts
index 2419606fa..bbfbf3234 100644
--- a/source/selectors/storageSelectors.ts
+++ b/source/selectors/storageSelectors.ts
@@ -1,10 +1,10 @@
-import { RootState } from 'Reducers/rootReducer'
+import { RootState, Simulation } from 'Reducers/rootReducer'
 
 // Note: it is currently not possible to define SavedSimulation as the return
 // type of the currentSimulationSelector function because the type would then
 // circulary reference itself.
 export type SavedSimulation = {
-	situation: RootState['simulation']['situation']
+	situation: Simulation['situation']
 	activeTargetInput: RootState['activeTargetInput']
 	foldedSteps: RootState['conversationSteps']['foldedSteps']
 }
@@ -13,21 +13,25 @@ export const currentSimulationSelector = (
 	state: RootState
 ): SavedSimulation => {
 	return {
-		situation: state.simulation.situation,
+		situation: state.simulation?.situation ?? {},
 		activeTargetInput: state.activeTargetInput,
 		foldedSteps: state.conversationSteps.foldedSteps
 	}
 }
 
-export const createStateFromSavedSimulation = (state: RootState) =>
-	state.previousSimulation && {
-		activeTargetInput: state.previousSimulation.activeTargetInput,
-		simulation: {
-			...state.simulation,
-			situation: state.previousSimulation.situation || {}
-		},
-		conversationSteps: {
-			foldedSteps: state.previousSimulation.foldedSteps
-		},
-		previousSimulation: null
-	}
+export const createStateFromSavedSimulation = (
+	state: RootState
+): Partial<RootState> =>
+	state.previousSimulation
+		? {
+				activeTargetInput: state.previousSimulation.activeTargetInput,
+				simulation: {
+					...state.simulation,
+					situation: state.previousSimulation.situation || {}
+				} as Simulation,
+				conversationSteps: {
+					foldedSteps: state.previousSimulation.foldedSteps
+				},
+				previousSimulation: null
+		  }
+		: {}
diff --git a/source/sites/mon-entreprise.fr/pages/Créer/AfterRegistration.tsx b/source/sites/mon-entreprise.fr/pages/Créer/AfterRegistration.tsx
index 6d335db70..d254784e4 100644
--- a/source/sites/mon-entreprise.fr/pages/Créer/AfterRegistration.tsx
+++ b/source/sites/mon-entreprise.fr/pages/Créer/AfterRegistration.tsx
@@ -15,7 +15,7 @@ export default function AfterRegistration() {
 		(state: RootState) => state.inFranceApp.companyStatusChoice
 	)
 	const { t } = useTranslation()
-	const isAutoentrepreneur = statutChoisi.match('auto-entrepreneur')
+	const isAutoentrepreneur = statutChoisi?.match('auto-entrepreneur')
 
 	return (
 		<Animate.fromBottom>
diff --git a/source/sites/mon-entreprise.fr/pages/Créer/GuideStatut/PreviousAnswers.tsx b/source/sites/mon-entreprise.fr/pages/Créer/GuideStatut/PreviousAnswers.tsx
index a0944eccf..5a951ac62 100644
--- a/source/sites/mon-entreprise.fr/pages/Créer/GuideStatut/PreviousAnswers.tsx
+++ b/source/sites/mon-entreprise.fr/pages/Créer/GuideStatut/PreviousAnswers.tsx
@@ -38,20 +38,21 @@ export default function PreviousAnswers() {
 	const legalStatus = useSelector(
 		(state: RootState) => state.inFranceApp.companyLegalStatus
 	)
+	if (Object.values(legalStatus).length < 1) {
+		return null
+	}
 	return (
-		!!Object.values(legalStatus).length && (
-			<ul css="margin-bottom: -1rem;">
-				{Object.entries(legalStatus).map(
-					([key, value]) =>
-						!isNil(value) && (
-							<li key={key}>
-								<Link to={sitePaths.créer.guideStatut[key]}>
-									{requirementToText(key as any, value as any)}
-								</Link>
-							</li>
-						)
-				)}
-			</ul>
-		)
+		<ul css="margin-bottom: -1rem;">
+			{Object.entries(legalStatus).map(
+				([key, value]) =>
+					!isNil(value) && (
+						<li key={key}>
+							<Link to={sitePaths.créer.guideStatut[key]}>
+								{requirementToText(key as any, value as any)}
+							</Link>
+						</li>
+					)
+			)}
+		</ul>
 	)
 }
diff --git a/source/sites/mon-entreprise.fr/pages/Dev/IntegrationTest.tsx b/source/sites/mon-entreprise.fr/pages/Dev/IntegrationTest.tsx
index f33c254c8..4c1216665 100644
--- a/source/sites/mon-entreprise.fr/pages/Dev/IntegrationTest.tsx
+++ b/source/sites/mon-entreprise.fr/pages/Dev/IntegrationTest.tsx
@@ -21,8 +21,10 @@ export default function IntegrationTest() {
 		script.src = window.location.origin + '/simulateur-iframe-integration.js'
 		script.dataset.module = currentModule
 		script.dataset.couleur = colour
-		domNode.current.innerHTML = ''
-		domNode.current.appendChild(script)
+		if (domNode.current) {
+			domNode.current.innerHTML = ''
+			domNode.current.appendChild(script)
+		}
 		// eslint-disable-next-line react-hooks/exhaustive-deps
 	}, [version])
 	return (
diff --git a/source/sites/mon-entreprise.fr/pages/Gérer/AideOrganismeLocal.tsx b/source/sites/mon-entreprise.fr/pages/Gérer/AideOrganismeLocal.tsx
index fa81314ad..8d3d5c44f 100644
--- a/source/sites/mon-entreprise.fr/pages/Gérer/AideOrganismeLocal.tsx
+++ b/source/sites/mon-entreprise.fr/pages/Gérer/AideOrganismeLocal.tsx
@@ -11,6 +11,7 @@ const aideMidiPyrenéesAutoEntrepreneur = (state: RootState) => {
 		return false
 	}
 	return (
+		company.dateDébutActivité &&
 		new Date(company.dateDébutActivité) > new Date('2019-04-01') &&
 		company.isAutoEntrepreneur &&
 		company.localisation &&
diff --git a/source/sites/mon-entreprise.fr/pages/Gérer/Home.tsx b/source/sites/mon-entreprise.fr/pages/Gérer/Home.tsx
index 731012109..f1c11baa2 100644
--- a/source/sites/mon-entreprise.fr/pages/Gérer/Home.tsx
+++ b/source/sites/mon-entreprise.fr/pages/Gérer/Home.tsx
@@ -20,7 +20,7 @@ import * as Animate from 'Ui/animate'
 import AideOrganismeLocal from './AideOrganismeLocal'
 import businessPlan from './businessPlan.svg'
 
-const infereRégimeFromCompanyDetails = (company: Company) => {
+const infereRégimeFromCompanyDetails = (company: Company | null) => {
 	if (!company) {
 		return null
 	}
@@ -28,14 +28,14 @@ const infereRégimeFromCompanyDetails = (company: Company) => {
 		return 'auto-entrepreneur'
 	}
 	if (
-		['EI', 'EURL'].includes(company.statutJuridique) ||
+		['EI', 'EURL'].includes(company.statutJuridique ?? '') ||
 		(company.statutJuridique === 'SARL' && company.isDirigeantMajoritaire)
 	) {
 		return 'indépendant'
 	}
 
 	if (
-		['SASU', 'SAS'].includes(company.statutJuridique) ||
+		['SASU', 'SAS'].includes(company.statutJuridique ?? '') ||
 		(company.statutJuridique === 'SARL' && !company.isDirigeantMajoritaire)
 	) {
 		return 'assimilé-salarié'
@@ -193,7 +193,7 @@ export default function SocialSecurity() {
 }
 
 type CompanySectionProps = {
-	company: Company
+	company: Company | null
 }
 
 const CompanySection = ({ company }: CompanySectionProps) => {
@@ -203,7 +203,7 @@ const CompanySection = ({ company }: CompanySectionProps) => {
 		false
 	)
 
-	const companyRef = useRef(null)
+	const companyRef = useRef<Company | null>(null)
 	useEffect(() => {
 		if (companyRef.current !== company) {
 			companyRef.current = company
diff --git a/source/sites/mon-entreprise.fr/pages/Simulateurs/ArtisteAuteur.tsx b/source/sites/mon-entreprise.fr/pages/Simulateurs/ArtisteAuteur.tsx
index 73372cf9d..e116c305d 100644
--- a/source/sites/mon-entreprise.fr/pages/Simulateurs/ArtisteAuteur.tsx
+++ b/source/sites/mon-entreprise.fr/pages/Simulateurs/ArtisteAuteur.tsx
@@ -147,12 +147,13 @@ function WarningRegimeSpecial() {
 	const situation = useSelector(situationSelector)
 	const recettes = situation['artiste-auteur . revenus . BNC . recettes']
 	const showWarning = recettes !== 0 && recettes >= 70000
+	if (!showWarning) {
+		return null
+	}
 	return (
-		showWarning && (
-			<li>
-				Vos revenus ne vous permettent pas d'opter pour le régime micro-BNC.
-			</li>
-		)
+		<li>
+			Vos revenus ne vous permettent pas d'opter pour le régime micro-BNC.
+		</li>
 	)
 }
 
@@ -179,23 +180,25 @@ function CotisationsResult() {
 		setDisplay(true)
 	}
 
+	if (!display) {
+		return null
+	}
+
 	return (
-		display && (
-			<Animate.appear>
-				<ResultBlock className="ui__ card">
-					<ResultLabel>Montant des cotisations</ResultLabel>
-					<RuleLink dottedName={cotisationRule.dottedName}>
-						{formatValue({
-							value: cotisationRule.nodeValue,
-							language: 'fr',
-							unit: '€',
-							maximumFractionDigits: 0
-						})}
-					</RuleLink>
-				</ResultBlock>
-				{cotisationRule.nodeValue ? <RepartitionCotisations /> : null}
-			</Animate.appear>
-		)
+		<Animate.appear>
+			<ResultBlock className="ui__ card">
+				<ResultLabel>Montant des cotisations</ResultLabel>
+				<RuleLink dottedName={cotisationRule.dottedName}>
+					{formatValue({
+						value: cotisationRule.nodeValue,
+						language: 'fr',
+						unit: '€',
+						maximumFractionDigits: 0
+					})}
+				</RuleLink>
+			</ResultBlock>
+			{cotisationRule.nodeValue ? <RepartitionCotisations /> : null}
+		</Animate.appear>
 	)
 }
 
diff --git a/source/utils.ts b/source/utils.ts
index 731ddb142..7749c953a 100644
--- a/source/utils.ts
+++ b/source/utils.ts
@@ -1,14 +1,14 @@
-export let capitalise0 = (name: string): string =>
+export let capitalise0 = (name: string | undefined) =>
 	name && name[0].toUpperCase() + name.slice(1)
 
-export function debounce<ArgType>(
-	timeout: number,
-	fn: (arg?: ArgType) => void
-): (arg?: ArgType) => void {
+export const debounce = <F extends (...args: any[]) => void>(
+	waitFor: number,
+	fn: F
+) => {
 	let timeoutId: ReturnType<typeof setTimeout>
-	return (...args) => {
+	return (...args: any[]) => {
 		clearTimeout(timeoutId)
-		timeoutId = setTimeout(() => fn(...args), timeout)
+		timeoutId = setTimeout(() => fn(...args), waitFor)
 	}
 }
 
diff --git a/tsconfig.json b/tsconfig.json
index f127a74bf..a70f455d0 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -16,9 +16,13 @@
 		// strictBindCallApply, strictNullChecks, strictFunctionTypes, and
 		// strictPropertyInitialization. During the transition we enable these
 		// settings one by one.
+		// Note: almost all parameters are now enabled. The only one remaining
+		// is noImplicitAny -- but it's a hard one.
 		"noImplicitThis": true,
 		"strictBindCallApply": true,
 		"strictFunctionTypes": true,
+		"strictNullChecks": true,
+		"strictPropertyInitialization": true,
 		"paths": {
 			"Actions/*": ["actions/*"],
 			"Components": ["components"],