diff --git a/core/lib/Drupal/Core/Test/EventSubscriber/HttpRequestSubscriber.php b/core/lib/Drupal/Core/Test/EventSubscriber/HttpRequestSubscriber.php
index 32d696d..8a950d2 100644
--- a/core/lib/Drupal/Core/Test/EventSubscriber/HttpRequestSubscriber.php
+++ b/core/lib/Drupal/Core/Test/EventSubscriber/HttpRequestSubscriber.php
@@ -16,6 +16,8 @@ class HttpRequestSubscriber {
 
   /**
    * {@inheritdoc}
+   *
+   * HTTP middleware that replaces the user agent for simpletest requests.
    */
   public function __invoke() {
     // If the database prefix is being used by SimpleTest to run the tests in a copied