diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index 18635a6..ecc150a 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -553,9 +553,6 @@ function watchdog_exception($type, Exception $exception, $message = NULL, $varia
  *   @endcode
  *   If there are no messages set, the function returns NULL.
  *
- * @throws \UnexpectedValueException
- *   When passed a render array with #attached.
- *
  * @see drupal_get_messages()
  * @see status-messages.html.twig
  */
@@ -577,10 +574,8 @@ function drupal_set_message($message = NULL, $type = 'status', $repeat = FALSE)
     if (is_array($message)) {
       $rendered_message = \Drupal::service('renderer')->renderPlain($message);
       // The rendering will have bubbled any #attached assets to the top level
-      // of $message. Throw an exception if there were any.
-      if (!empty($message['#attached'])) {
-        throw new \UnexpectedValueException('Render arrays passed to drupal_set_message() cannot have attached assets.');
-      }
+      // of $message. Assert that there are none.
+      assert('empty($message[\'#attached\']);', 'Render arrays passed to drupal_set_message() cannot have attached assets.');
       $message = $rendered_message;
     }
 
diff --git a/core/modules/system/src/Tests/Bootstrap/DrupalSetMessageTest.php b/core/modules/system/src/Tests/Bootstrap/DrupalSetMessageTest.php
index 85a66e0..e345f37 100644
--- a/core/modules/system/src/Tests/Bootstrap/DrupalSetMessageTest.php
+++ b/core/modules/system/src/Tests/Bootstrap/DrupalSetMessageTest.php
@@ -59,25 +59,20 @@ function testDrupalSetMessage() {
     // Ensure that strings that are not marked as safe are escaped.
     $this->assertEscaped('<em>This<span>markup will be</span> escaped</em>.');
 
-    // Test render arrays that incorrectly add #attached.
-    try {
-      drupal_set_message(['#attached' => ['drupalSettings' => ['pecary' => TRUE]]]);
-      $this->fail('Render arrays passed to drupal_set_message() cannot have attached assets.');
-    }
-    catch (\UnexpectedValueException $e) {
-      $this->pass('Render arrays passed to drupal_set_message() cannot have attached assets.');
-    }
+    // Ensure render arrays fail with #attached when assertions are enabled.
+    $this->drupalGet('system-test/drupal-set-message-fail');
+    $this->assertText('Render arrays passed to drupal_set_message() cannot have attached assets.');
+  }
 
-    // Test render arrays that incorrectly add #attached in a child element.
-    try {
-      $array_with_attachments['foo']['bar']['#markup'] = t('Render array with assets');
-      $array_with_attachments['foo']['bar']['#attached']['drupalSettings'] = ['pecary' => TRUE];
-      drupal_set_message($array_with_attachments);
-      $this->fail('Render arrays passed to drupal_set_message() cannot have attached assets in child elements.');
-    }
-    catch (\UnexpectedValueException $e) {
-      $this->pass('Render arrays passed to drupal_set_message() cannot have attached assets in child elements.');
+  /**
+   * {@inheritdoc}
+   */
+  protected function error($message = '', $group = 'Other', array $caller = NULL) {
+    // @todo Clean this up in https://www.drupal.org/node/2536560.
+    if (strpos($message, 'Render arrays passed to drupal_set_message() cannot have attached assets.') !== FALSE) {
+      // We're expecting this error.
+      return FALSE;
     }
+    parent::error($message, $group, $caller);
   }
-
 }
diff --git a/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php b/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php
index 63cd091..ddb1a83 100644
--- a/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php
+++ b/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php
@@ -148,6 +148,26 @@ public function drupalSetMessageTest() {
   }
 
   /**
+   * Tests setting messages with a render array containing #attached.
+   *
+   * @return array
+   *   Empty array, we just test the setting of messages.
+   */
+  public function drupalSetMessageFailTest() {
+    // Enable assert testing.
+    // @todo Clean this up in https://www.drupal.org/node/2536560.
+    $old_assert_active = assert_options(ASSERT_ACTIVE, 1);
+
+    $array_with_attachments['foo']['bar']['#markup'] = t('Render array with assets');
+    $array_with_attachments['foo']['bar']['#attached']['drupalSettings'] = ['pecary' => TRUE];
+    drupal_set_message($array_with_attachments);
+
+    // Reset assert settings.
+    assert_options(ASSERT_ACTIVE, $old_assert_active);
+    return [];
+  }
+
+  /**
    * Controller to return $_GET['destination'] for testing.
    *
    * @param \Symfony\Component\HttpFoundation\Request $request
diff --git a/core/modules/system/tests/modules/system_test/system_test.routing.yml b/core/modules/system/tests/modules/system_test/system_test.routing.yml
index ecb9921..8062111 100644
--- a/core/modules/system/tests/modules/system_test/system_test.routing.yml
+++ b/core/modules/system/tests/modules/system_test/system_test.routing.yml
@@ -21,6 +21,14 @@ system_test.drupal_set_message:
   requirements:
     _access: 'TRUE'
 
+system_test.drupal_set_message.fail:
+  path: '/system-test/drupal-set-message-fail'
+  defaults:
+    _title: 'Uses #attached in render array with drupal_set_message()'
+    _controller: '\Drupal\system_test\Controller\SystemTestController::drupalSetMessageFailTest'
+  requirements:
+    _access: 'TRUE'
+
 system_test.main_content_fallback:
   path: '/system-test/main-content-fallback'
   defaults: