diff --git a/src/Forms/Validation/RequiredFieldsValidator.php b/src/Forms/Validation/RequiredFieldsValidator.php
index 48d7aad59ff..170fa5d3bed 100644
--- a/src/Forms/Validation/RequiredFieldsValidator.php
+++ b/src/Forms/Validation/RequiredFieldsValidator.php
@@ -250,7 +250,7 @@ public function appendRequiredFields($requiredFields)
      */
     public function fieldIsRequired($fieldName)
     {
-        return isset($this->required[$fieldName]);
+        return isset($this->required[$fieldName ?? '']);
     }
 
     /**