diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index ced9c4c..99160be 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -517,9 +517,9 @@ function watchdog_exception($type, Exception $exception, $message = NULL, $varia
  * @param string|array|\Drupal\Component\Utility\SafeStringInterface $message
  *   (optional) The translated message to be displayed to the user. For
  *   consistency with other messages, it should begin with a capital letter and
- *   end with a period. A render array may also be provided for markup, but
- *   #attached is not supported. Additionally, any cache tags and contexts are
- *   ignored, however this is okay because page caching is disabled.
+ *   end with a period. A render array may also be provided for markup, and if
+ *   so it will be rendered with RendererInterface::renderPlain() (meaning
+ *   caching and attachment data will be stripped).
  * @param string $type
  *   (optional) The message's type. Defaults to 'status'. These values are
  *   supported:
@@ -556,6 +556,7 @@ function watchdog_exception($type, Exception $exception, $message = NULL, $varia
  *
  * @see drupal_get_messages()
  * @see status-messages.html.twig
+ * @see \Drupal\Core\Render\RendererInterface::renderPlain()
  */
 function drupal_set_message($message = NULL, $type = 'status', $repeat = FALSE) {
   if (isset($message)) {
@@ -573,11 +574,7 @@ function drupal_set_message($message = NULL, $type = 'status', $repeat = FALSE)
     // may have unpredictable results and some render array functionality
     // (like assets) should not be supported.
     if (is_array($message)) {
-      $rendered_message = \Drupal::service('renderer')->renderPlain($message);
-      // The rendering will have all attachments bubbled to #attached in the top
-      // level of $message. Assert that there are none.
-      assert('empty($message[\'#attached\']);', 'Render arrays passed to drupal_set_message() cannot have attachments.');
-      $message = $rendered_message;
+      $message = \Drupal::service('renderer')->renderPlain($message);
     }
 
     // Do not use strict type checking so that equivalent string and