diff --git a/core/lib/Drupal/Core/EventSubscriber/FormControllerSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/FormControllerSubscriber.php
index 8a878c5..ecfb865 100644
--- a/core/lib/Drupal/Core/EventSubscriber/FormControllerSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/FormControllerSubscriber.php
@@ -70,7 +70,7 @@ public function onKernelController(FilterControllerEvent $event) {
         $arguments = $this->controllerResolver->getArguments($request, $original_controller);
         $response = call_user_func_array($original_controller, $arguments);
         if (!$response instanceof FormAndFormState) {
-          throw new \LogicException('asdf');
+          throw new \LogicException('Form controllers must return an instance of \Drupal\Core\Form\FormAndFormState');
         }
 
         $form = $response->getForm();
diff --git a/core/tests/Drupal/Tests/Core/EventSubscriber/FormControllerSubscriberTest.php b/core/tests/Drupal/Tests/Core/EventSubscriber/FormControllerSubscriberTest.php
index 3e3419c..2576b55 100644
--- a/core/tests/Drupal/Tests/Core/EventSubscriber/FormControllerSubscriberTest.php
+++ b/core/tests/Drupal/Tests/Core/EventSubscriber/FormControllerSubscriberTest.php
@@ -174,7 +174,7 @@ public function testOnKernelControllerMismatchedBuildId() {
    * @covers ::onKernelController
    *
    * @expectedException \LogicException
-   * @expectedExceptionMessage qwer
+   * @expectedExceptionMessage Form controllers must return an instance of \Drupal\Core\Form\FormAndFormState
    */
   public function testOnKernelControllerException() {
     $request = new Request([FormControllerSubscriber::MAGIC_STRING_TO_BE_RENAMED => 'drupal_ajax_post']);