diff --git a/src/Builder/FormContractor.php b/src/Builder/FormContractor.php
index 3a6788032..f81b9f03a 100644
--- a/src/Builder/FormContractor.php
+++ b/src/Builder/FormContractor.php
@@ -196,8 +196,12 @@ private function hasAssociation(FieldDescriptionInterface $fieldDescription): bo
     /**
      * @param string[] $classes
      */
-    private function isAnyInstanceOf(string $type, array $classes): bool
+    private function isAnyInstanceOf(?string $type, array $classes): bool
     {
+        if (null === $type) {
+            return false;
+        }
+
         foreach ($classes as $class) {
             if (is_a($type, $class, true)) {
                 return true;