diff --git a/includes/form.inc b/includes/form.inc
index 3ef32ab..3c1584e 100644
--- a/includes/form.inc
+++ b/includes/form.inc
@@ -721,6 +721,9 @@ function drupal_form_submit($form_id, &$form_state) {
 function drupal_retrieve_form($form_id, &$form_state) {
   $forms = &drupal_static(__FUNCTION__);
 
+  // Record the $form_id.
+  $form_state['build_info']['form_id'] = $form_id;
+
   // Record the filepath of the include file containing the original form, so
   // the form builder callbacks can be loaded when the form is being rebuilt
   // from cache on a different path (such as 'system/ajax'). See