core/lib/Drupal/Core/Controller/HtmlControllerBase.php                | 4 ++--
 .../tests/modules/system_test/src/Controller/SystemTestController.php | 3 ++-
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/core/lib/Drupal/Core/Controller/HtmlControllerBase.php b/core/lib/Drupal/Core/Controller/HtmlControllerBase.php
index 3bc6957..7c84167 100644
--- a/core/lib/Drupal/Core/Controller/HtmlControllerBase.php
+++ b/core/lib/Drupal/Core/Controller/HtmlControllerBase.php
@@ -49,7 +49,7 @@ public function __construct(TitleResolverInterface $title_resolver, RenderHtmlRe
   /**
    * Converts a render array into an HtmlFragment object.
    *
-   * @param array|\Drupal\Core\Page\HtmlFragment $page_content
+   * @param array|\Drupal\Core\Page\HtmlFragment|\Symfony\Component\HttpFoundation\Response $page_content
    *   The page content area to display.
    * @param \Symfony\Component\HttpFoundation\Request $request
    *   The request object.
@@ -66,7 +66,7 @@ protected function createHtmlFragment($page_content, Request $request) {
     }
 
     if (is_string($page_content)) {
-      throw new \LogicException('_content controllers may not return strings.');
+      throw new \InvalidArgumentException('_content controllers may not return strings.');
     }
 
     $fragment = $this->renderHtmlRenderer->render($page_content);
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 9906b10..d79e3f6 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
@@ -154,8 +154,9 @@ public function shutdownFunctions($arg1, $arg2) {
     // @see _drupal_shutdown_function()
     // @see \Drupal\system\Tests\System\ShutdownFunctionsTest
     if (function_exists('fastcgi_finish_request')) {
-      return 'The function fastcgi_finish_request exists when serving the request.';
+      return ['#markup' => 'The function fastcgi_finish_request exists when serving the request.'];
     }
+    return [];
   }
 
   /**