diff --git a/core/lib/Drupal/Core/Controller/HtmlControllerBase.php b/core/lib/Drupal/Core/Controller/HtmlControllerBase.php
index 87d7956..88e0efe 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|\Symfony\Component\HttpFoundation\Response $page_content
+   * @param array|\Drupal\Core\Page\HtmlFragmentInterface|\Symfony\Component\HttpFoundation\Response $page_content
    *   The page content area to display.
    * @param \Symfony\Component\HttpFoundation\Request $request
    *   The request object.
@@ -58,6 +58,7 @@ public function __construct(TitleResolverInterface $title_resolver, RenderHtmlRe
    *   A page object.
    *
    * @throws \InvalidArgumentException
+   *   Thrown if the controller returns a string.
    */
   protected function createHtmlFragment($page_content, Request $request) {
     // Allow controllers to return a HtmlFragment or a Response object directly.
@@ -66,7 +67,7 @@ protected function createHtmlFragment($page_content, Request $request) {
     }
 
     if (is_string($page_content)) {
-      throw new \InvalidArgumentException('_content controllers may not return strings.');
+      throw new \InvalidArgumentException('_content controllers are not allowed to return strings. You can return a render array, a html fragment or a response object.');
     }
 
     $fragment = $this->renderHtmlRenderer->render($page_content);
diff --git a/core/modules/system/src/Tests/Routing/RouterTest.php b/core/modules/system/src/Tests/Routing/RouterTest.php
index 4fd44bf..c9e70ca 100644
--- a/core/modules/system/src/Tests/Routing/RouterTest.php
+++ b/core/modules/system/src/Tests/Routing/RouterTest.php
@@ -44,7 +44,7 @@ public function testDefaultController() {
     $this->assertRaw('</html>', 'Page markup was found.');
 
     // In some instances, the subrequest handling may get confused and render
-    // a page inception style.  This test verifies that is not happening.
+    // a page inception style.  This test verifies that is not happening
     $this->assertNoPattern('#</body>.*</body>#s', 'There was no double-page effect from a misrendered subrequest.');
   }