diff --git a/source/components/conversation/select/SelectGéo.js b/source/components/conversation/select/SelectGéo.js
index ec3f704db..2ef07ff2a 100644
--- a/source/components/conversation/select/SelectGéo.js
+++ b/source/components/conversation/select/SelectGéo.js
@@ -42,13 +42,29 @@ export default FormDecorator('select')(
 					submit
 				} = this.props,
 				submitOnChange = option => {
-					tauxVersementTransport(option.code).then(({ taux }) => {
-						// serialize to not mix our data schema and the API response's
-						onChange(
-							JSON.stringify({ ...option, 'taux du versement transport': taux })
-						)
-						submit()
-					})
+					tauxVersementTransport(option.code)
+						.then(({ taux }) => {
+							// serialize to not mix our data schema and the API response's
+							onChange(
+								JSON.stringify({
+									...option,
+									...(taux != undefined
+										? {
+												'taux du versement transport': taux
+										  }
+										: {})
+								})
+							)
+							submit()
+						})
+						.catch(error => {
+							//eslint-disable-next-line no-console
+							console.log(
+								'Erreur dans la récupération du taux de versement transport à partir du code commune',
+								error
+							) || onChange(JSON.stringify({ option }))
+							submit() // eslint-disable-line no-console
+						})
 				}
 
 			return (
diff --git a/source/engine/mecanisms.js b/source/engine/mecanisms.js
index 5be3dedc8..9c1c79bc6 100644
--- a/source/engine/mecanisms.js
+++ b/source/engine/mecanisms.js
@@ -55,11 +55,7 @@ import InversionNumérique from './mecanismViews/InversionNumérique'
 import Product from './mecanismViews/Product'
 import Somme from './mecanismViews/Somme'
 import Variations from './mecanismViews/Variations'
-import {
-	disambiguateRuleReference,
-	findRuleByDottedName,
-	findRuleByName
-} from './rules'
+import { disambiguateRuleReference, findRuleByDottedName } from './rules'
 import { anyNull, val } from './traverse-common-functions'
 import uniroot from './uniroot'
 
@@ -859,14 +855,21 @@ export let mecanismSynchronisation = (recurse, k, v) => {
 			node.explanation.API
 		)
 
+		let valuePath = v.chemin.split(' . ')
+
 		let nodeValue =
-			val(APIExplanation) == null
-				? null
-				: path(v.chemin.split(' . '))(val(APIExplanation))
+			val(APIExplanation) == null ? null : path(valuePath, val(APIExplanation))
+
+		// If the API gave a non null value, then some of its props may be null (the API can be composed of multiple API, some failing). Then this prop will be set to the default value defined in the API's rule
+		let safeNodeValue =
+			nodeValue == null && val(APIExplanation) != null
+				? path(valuePath, APIExplanation.explanation.defaultValue)
+				: nodeValue
+
 		let missingVariables =
 			val(APIExplanation) === null ? { [APIExplanation.dottedName]: 1 } : {}
 		let explanation = { ...v, API: APIExplanation }
-		return rewriteNode(node, nodeValue, explanation, missingVariables)
+		return rewriteNode(node, safeNodeValue, explanation, missingVariables)
 	}
 
 	return {