diff --git a/core/modules/file/file.module b/core/modules/file/file.module
index c951807..bc2633b 100644
--- a/core/modules/file/file.module
+++ b/core/modules/file/file.module
@@ -259,7 +259,14 @@ function file_ajax_upload() {
   // Get the current element and count the number of files.
   $current_element = $form;
   foreach ($form_parents as $parent) {
-    $current_element = $current_element[$parent];
+    // Make sure the path is set: custom form elements might not have added it yet.
+    if (isset($current_element[$parent])) {
+      $current_element = $current_element[$parent];
+    }
+    else {
+      $current_element = NULL;
+      break;
+    }
   }
   $current_file_count = isset($current_element['#file_upload_delta']) ? $current_element['#file_upload_delta'] : 0;