diff --git a/src/Doctrine/DoctrineHelper.php b/src/Doctrine/DoctrineHelper.php
index 12b58d309..961d42981 100644
--- a/src/Doctrine/DoctrineHelper.php
+++ b/src/Doctrine/DoctrineHelper.php
@@ -311,7 +311,7 @@ private function getMappingDriverForNamespace(string $namespace): ?MappingDriver
         $lowestCharacterDiff = null;
         $foundDriver = null;
 
-        foreach ($this->mappingDriversByPrefix as $key => $mappings) {
+        foreach ($this->mappingDriversByPrefix ?? [] as $mappings) {
             foreach ($mappings as [$prefix, $driver]) {
                 $diff = substr_compare($namespace, $prefix, 0);