diff --git a/core/lib/Drupal/Core/Messenger/Messenger.php b/core/lib/Drupal/Core/Messenger/Messenger.php
index 073ebc4..f7dac0b 100644
--- a/core/lib/Drupal/Core/Messenger/Messenger.php
+++ b/core/lib/Drupal/Core/Messenger/Messenger.php
@@ -19,6 +19,9 @@ class Messenger implements MessengerInterface {
    * {@inheritdoc}
    */
   public function addMessage($message, $type = self::STATUS, $repeat = FALSE) {
+    if (!isset($_SESSION)) {
+      $_SESSION = array();
+    }
     if ($repeat || !array_key_exists('messages', $_SESSION) || !array_key_exists($type, $_SESSION['messages']) || !in_array($message, $_SESSION['messages'][$type])) {
       $_SESSION['messages'][$type][] = $message;
     }