diff --git a/core/lib/Drupal/Core/Form/FormState.php b/core/lib/Drupal/Core/Form/FormState.php
index 9d651cd..bd80d49 100644
--- a/core/lib/Drupal/Core/Form/FormState.php
+++ b/core/lib/Drupal/Core/Form/FormState.php
@@ -211,6 +211,7 @@ class FormState implements FormStateInterface {
     'form_id',
     'form_token',
     'form_build_id',
+    'form_build_key',
     'op',
   ];