diff -u b/core/misc/ajax.js b/core/misc/ajax.js
--- b/core/misc/ajax.js
+++ b/core/misc/ajax.js
@@ -526,7 +526,7 @@
     else {
       ajax.options.url += '&';
     }
-    ajax.options.url += Drupal.Ajax.WRAPPER_FORMAT + '=drupal_' + (element_settings.dialogType || 'ajax');
+    ajax.options.url += Drupal.ajax.WRAPPER_FORMAT + '=drupal_' + (element_settings.dialogType || 'ajax');
 
     // Bind the ajaxSubmit function to the element event.
     $(ajax.element).on(element_settings.event, function (event) {
@@ -563,7 +563,7 @@
    *
    * @default
    */
-  Drupal.Ajax.WRAPPER_FORMAT = '_wrapper_format';
+  Drupal.ajax.WRAPPER_FORMAT = '_wrapper_format';
 
   /**
    * Request parameter to indicate that a request is a Drupal Ajax request.