diff -u b/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
--- b/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -316,7 +316,7 @@
 
     // Persisting $form_state is a side-effect disallowed during a "safe" HTTP
     // method (http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9.1.1).
-    if ($this->requestStack->getCurrentRequest()->isMethodSafe()) {
+    if (!$this->requestStack->getCurrentRequest()->isMethodSafe()) {
       $form_state->setCached();
     }