diff --git a/core/includes/form.inc b/core/includes/form.inc
index af1950a..9cad700 100644
--- a/core/includes/form.inc
+++ b/core/includes/form.inc
@@ -4100,7 +4100,7 @@ function form_process_autocomplete($element, &$form_state) {
     // @todo Refactor autocomplete.js to accept Drupal.settings instead of
     //   requiring extraneous markup.
     $element['autocomplete'] = array(
-      '#theme' => 'hidden',
+      '#type' => 'hidden',
       '#attributes' => array(
         'id' => $element['#id'] . '-autocomplete',
         'value' => url($element['#autocomplete_path'], array('absolute' => TRUE)),