diff --git a/extension/Classes/Core/Form/FormAsFile.php b/extension/Classes/Core/Form/FormAsFile.php
index a2e334f248d701f05c861475182efcb5a6ead2ec..09ae07e3f0a0dc42f9d04ff0e416d6a8d9aa077d 100644
--- a/extension/Classes/Core/Form/FormAsFile.php
+++ b/extension/Classes/Core/Form/FormAsFile.php
@@ -684,7 +684,7 @@ class FormAsFile
 
             // Replace container id references with name references
             $containerId = $formElement[FE_ID_CONTAINER];
-            if ($containerId !== 0) {
+            if ($containerId !== 0 && array_key_exists($containerId, $containerNames)) {
                 $formElement[FE_FILE_CONTAINER_NAME] = $containerNames[$containerId];
             }