diff --git a/libraries/src/Access/Access.php b/libraries/src/Access/Access.php
index 059be212e4835..2452c86673710 100644
--- a/libraries/src/Access/Access.php
+++ b/libraries/src/Access/Access.php
@@ -531,6 +531,12 @@ public static function getAssetRules($assetKey, $recursive = false, $recursivePa
 
 				foreach ($ancestors as $id)
 				{
+					// There are no rules for this ancestor
+					if (!isset(self::$assetPermissionsParentIdMapping[$extensionName][$id]))
+					{
+						continue;
+					}
+
 					// If full recursive mode, but not recursive parent mode, do not add the extension asset rules.
 					if ($recursive && !$recursiveParentAsset && self::$assetPermissionsParentIdMapping[$extensionName][$id]->name === $extensionName)
 					{