diff -u b/src/Controller/Search404Controller.php b/src/Controller/Search404Controller.php
--- b/src/Controller/Search404Controller.php
+++ b/src/Controller/Search404Controller.php
@@ -155,7 +155,7 @@
       }
 
       // Construct the search form.
-      $build['search_form'] = $build['search_form'] = $this->formBuilder()->getForm(SearchPageForm::class, $entity);
+      $build['search_form'] = $this->formBuilder()->getForm(SearchPageForm::class, $entity);
 
       // Set the custom page text on the top of the results.
       $search_404_page_text = \Drupal::config('search404.settings')->get('search404_page_text');