diff --git a/core/lib/Drupal/Core/DependencyInjection/Container.php b/core/lib/Drupal/Core/DependencyInjection/Container.php
index 4e28d54..b5a7edb 100644
--- a/core/lib/Drupal/Core/DependencyInjection/Container.php
+++ b/core/lib/Drupal/Core/DependencyInjection/Container.php
@@ -30,7 +30,7 @@ public function set($id, $service, $scope = SymfonyContainer::SCOPE_CONTAINER) {
    * {@inheritdoc}
    */
   public function __sleep() {
-    trigger_error('The container was serialized.', E_USER_ERROR);
+    assert(FALSE, 'The container was serialized.');
     return array_keys(get_object_vars($this));
   }