diff --git a/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
index 6d549ad..377a4e9 100644
--- a/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -390,7 +390,7 @@ public function setCache($form_build_id, $form, FormStateInterface $form_state)
     // the one passed in as a parameter. This is an additional safety measure to
     // prevent legacy code operating directly with form_get_cache and
     // form_set_cache from accidentally overwriting immutable form state.
-    if ($form['#build_id'] != $form_build_id) {
+    if (isset($form['#build_id']) && $form['#build_id'] != $form_build_id) {
       $this->logger->get('form')->log(WATCHDOG_ERROR, 'Form build-id mismatch detected while attempting to store a form in the cache.', array());
       return;
     }
diff --git a/core/modules/system/tests/modules/form_test/form_test.routing.yml b/core/modules/system/tests/modules/form_test/form_test.routing.yml
index 70856de..ae2b989 100644
--- a/core/modules/system/tests/modules/form_test/form_test.routing.yml
+++ b/core/modules/system/tests/modules/form_test/form_test.routing.yml
@@ -429,10 +429,10 @@ form_test.two_instances:
 form_test.storage_legacy_handler:
   path: '/form-test/form-storage-legacy/{form_build_id}'
   defaults:
-    _content: '\Drupal\form_test\Controller\FormTestController::storageLegacyHandler'
+    _controller: '\Drupal\form_test\Controller\FormTestController::storageLegacyHandler'
     form_build_id: NULL
   requirements:
-    _permission: 'access content'
+    _access: 'TRUE'
 
 form_test.form_storage_page_cache:
   path: '/form-test/form-storage-page-cache'