diff --git a/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
index a77632f..52b24fd 100644
--- a/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -691,7 +691,7 @@ protected function buildFormAction() {
     // @todo Remove this unset() once these are removed from the request in
     //   https://www.drupal.org/node/2504709.
     $parsed = UrlHelper::parse($request_uri);
-    unset($parsed['query'][FormBuilderInterface::AJAX_FORM_REQUEST], $parsed['query'][MainContentViewSubscriber::WRAPPER_FORMAT]);
+    unset($parsed['query'][static::AJAX_FORM_REQUEST], $parsed['query'][MainContentViewSubscriber::WRAPPER_FORMAT]);
     return $parsed['path'] . ($parsed['query'] ? ('?' . UrlHelper::buildQuery($parsed['query'])) : '');
   }