diff --git a/core/lib/Drupal/Core/HttpKernel/TestMiddleware.php b/core/lib/Drupal/Core/HttpKernel/TestMiddleware.php
index 88c076d..7513077 100644
--- a/core/lib/Drupal/Core/HttpKernel/TestMiddleware.php
+++ b/core/lib/Drupal/Core/HttpKernel/TestMiddleware.php
@@ -11,14 +11,25 @@
 use Symfony\Component\HttpFoundation\Response;
 use Symfony\Component\HttpKernel\HttpKernelInterface;
 
+/**
+ * Provides an example middleware.
+ */
 class TestMiddleware Implements HttpKernelInterface {
 
   /**
+   * The wrapped HTTP kernel.
+   *
    * @var \Symfony\Component\HttpKernel\HttpKernelInterface
    */
   protected $kernel;
 
-  public function __construct(HttpKernelInterface $kernel) {
+  /**
+   * Constructs a new TestMiddleware object.
+   *
+   * @param \Symfony\Component\HttpKernel\HttpKernelInterface $kernel
+   *   The wrapped HTTP kernel.
+   */
+  public function __construct(HttpKernelInterface $kernel = NULL) {
     $this->kernel = $kernel;
   }
 
@@ -31,4 +42,3 @@ public function handle(Request $request, $type = self::MASTER_REQUEST, $catch =
   }
 
 }
-