core/lib/Drupal/Core/Form/FormBuilder.php | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
index ba8c055..7ba808b 100644
--- a/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -647,9 +647,7 @@ public function prepareForm($form_id, &$form, FormStateInterface &$form_state) {
 
     // Only update the action if it is not already set.
     if (!isset($form['#action'])) {
-      $form['#action'] = $this->buildFormAction();
-      $form['#cache']['contexts'][] = 'url.path';
-      $form['#cache']['contexts'][] = 'url.query_args';
+      $form = $this->buildFormAction($form);
     }
 
     // Fix the form method, if it is 'get' in $form_state, but not in $form.
@@ -759,10 +757,13 @@ public function prepareForm($form_id, &$form, FormStateInterface &$form_state) {
   /**
    * Builds the $form['#action'].
    *
-   * @return string
-   *   The URL to be used as the $form['#action'].
+   * @param array $form
+   *   An associative array containing the structure of the form.
+   *
+   * @return array
+   *   The updated form array.
    */
-  protected function buildFormAction() {
+  protected function buildFormAction($form) {
     // @todo Use <current> instead of the master request in
     //   https://www.drupal.org/node/2505339.
     $request = $this->requestStack->getMasterRequest();
@@ -778,7 +779,13 @@ protected function buildFormAction() {
     //   https://www.drupal.org/node/2504709.
     $parsed = UrlHelper::parse($request_uri);
     unset($parsed['query'][static::AJAX_FORM_REQUEST], $parsed['query'][MainContentViewSubscriber::WRAPPER_FORMAT]);
-    return $parsed['path'] . ($parsed['query'] ? ('?' . UrlHelper::buildQuery($parsed['query'])) : '');
+    $action = $parsed['path'] . ($parsed['query'] ? ('?' . UrlHelper::buildQuery($parsed['query'])) : '');
+
+    $form['#action'] = $action;
+    $form['#cache']['contexts'][] = 'url.path';
+    $form['#cache']['contexts'][] = 'url.query_args';
+
+    return $form;
   }
 
   /**