diff --git a/core/includes/form.inc b/core/includes/form.inc
index 32309a1882..1db96220ad 100644
--- a/core/includes/form.inc
+++ b/core/includes/form.inc
@@ -773,9 +773,6 @@ function batch_set($batch_definition) {
       }
       $batch_set['appended_after_index'] = $append_after_index;
 
-      $new_set_index = count($batch['sets']);
-      $new_sets = [];
-
       // Iterate by reference over the existing batch sets and assign them by
       // reference in the new batch sets array in order not to break a retrieved
       // reference to the current set. Among other places a reference to the
@@ -787,9 +784,11 @@ function batch_set($batch_definition) {
       // using not a new, but an existing and already used index for the new
       // batch set.
       // @see _batch_populate_queue().
+      $new_sets = [];
       foreach ($batch['sets'] as $index => &$set) {
         $new_sets[$index] = &$set;
         if ($index == $append_after_index) {
+          $new_set_index = count($batch['sets']);
           $new_sets[$new_set_index] = $batch_set;
         }
       }