diff --git a/core/lib/Drupal/Core/Form/FormCache.php b/core/lib/Drupal/Core/Form/FormCache.php
index b1b278f..3b7f706 100644
--- a/core/lib/Drupal/Core/Form/FormCache.php
+++ b/core/lib/Drupal/Core/Form/FormCache.php
@@ -153,7 +153,7 @@ public function getCache($form_build_id, FormStateInterface $form_state) {
    */
   protected function loadCachedFormState($form_build_id, FormStateInterface $form_state) {
     $session = $this->requestStack->getCurrentRequest()->getSession();
-    if ($session->has('form_state:' . $form_build_id) && $session_form_state = $session->get('form_state:' . $form_build_id)) {
+    if ($session->has('form_state:' . $form_build_id) && $session_form_state = unserialize($session->get('form_state:' . $form_build_id))) {
       if ($session_form_state['expires'] < $this->requestStack->getMasterRequest()->server->get('REQUEST_TIME')) {
         return;
       }
@@ -218,10 +218,10 @@ public function setCache($form_build_id, $form, FormStateInterface $form_state)
 
     if ($data = $form_state->getCacheableArray()) {
       $session = $this->requestStack->getCurrentRequest()->getSession();
-      $session->set('form_state:' . $form_build_id, [
+      $session->set('form_state:' . $form_build_id, serialize([
         'expires' => $this->requestStack->getMasterRequest()->server->get('REQUEST_TIME') + $expire,
         'data' => $data,
-      ]);
+      ]));
     }
   }
 
@@ -231,6 +231,8 @@ public function setCache($form_build_id, $form, FormStateInterface $form_state)
   public function deleteCache($form_build_id) {
     $this->keyValueExpirableFactory->get('form')->delete($form_build_id);
     $this->keyValueExpirableFactory->get('form_state')->delete($form_build_id);
+    $session = $this->requestStack->getMasterRequest()->getSession();
+    $session->remove('form_state:' . $form_build_id);
   }
 
 }
diff --git a/index.php b/index.php
index 53392f9..3202bac 100644
--- a/index.php
+++ b/index.php
@@ -8,6 +8,7 @@
  * See COPYRIGHT.txt and LICENSE.txt files in the "core" directory.
  */
 
+ini_set('display_errors', 1);
 use Drupal\Core\DrupalKernel;
 use Drupal\Core\Site\Settings;
 use Symfony\Component\HttpKernel\Exception\HttpExceptionInterface;