diff --git a/core/lib/Drupal/Core/DependencyInjection/Container.php b/core/lib/Drupal/Core/DependencyInjection/Container.php
index b4cac53..c86aace 100644
--- a/core/lib/Drupal/Core/DependencyInjection/Container.php
+++ b/core/lib/Drupal/Core/DependencyInjection/Container.php
@@ -31,7 +31,7 @@ public function set($id, $service, $scope = ContainerInterface::SCOPE_CONTAINER)
    * {@inheritdoc}
    */
   public function __sleep() {
-    trigger_error('The container was serialized.', E_USER_ERROR);
+    assert(FALSE, 'The container was serialized.');
     return array_keys(get_object_vars($this));
   }
 
diff --git a/core/lib/Drupal/Core/DependencyInjection/ContainerBuilder.php b/core/lib/Drupal/Core/DependencyInjection/ContainerBuilder.php
index ce692b5..e00426f 100644
--- a/core/lib/Drupal/Core/DependencyInjection/ContainerBuilder.php
+++ b/core/lib/Drupal/Core/DependencyInjection/ContainerBuilder.php
@@ -118,7 +118,7 @@ protected function callMethod($service, $call) {
    * {@inheritdoc}
    */
   public function __sleep() {
-    trigger_error('The container was serialized.', E_USER_ERROR);
+    assert(FALSE, 'The container was serialized.');
     return array_keys(get_object_vars($this));
   }
 
diff --git a/core/tests/Drupal/Tests/Core/DependencyInjection/ContainerBuilderTest.php b/core/tests/Drupal/Tests/Core/DependencyInjection/ContainerBuilderTest.php
index 0df17c4..157349d 100644
--- a/core/tests/Drupal/Tests/Core/DependencyInjection/ContainerBuilderTest.php
+++ b/core/tests/Drupal/Tests/Core/DependencyInjection/ContainerBuilderTest.php
@@ -70,4 +70,14 @@ public function testRegisterException() {
     $container->register('Bar');
   }
 
+  /**
+   * Tests serialization.
+   *
+   * @expectedException \AssertionError
+   */
+  public function testSerialize() {
+    $container = new ContainerBuilder();
+    serialize($container);
+  }
+
 }
diff --git a/core/tests/Drupal/Tests/Core/DependencyInjection/ContainerTest.php b/core/tests/Drupal/Tests/Core/DependencyInjection/ContainerTest.php
index 377f7be..ffbb591 100644
--- a/core/tests/Drupal/Tests/Core/DependencyInjection/ContainerTest.php
+++ b/core/tests/Drupal/Tests/Core/DependencyInjection/ContainerTest.php
@@ -20,7 +20,7 @@ class ContainerTest extends UnitTestCase {
   /**
    * Tests serialization.
    *
-   * @expectedException \PHPUnit_Framework_Error
+   * @expectedException \AssertionError
    */
   public function testSerialize() {
     $container = new Container();