diff -u b/core/lib/Drupal/Core/Render/Renderer.php b/core/lib/Drupal/Core/Render/Renderer.php
--- b/core/lib/Drupal/Core/Render/Renderer.php
+++ b/core/lib/Drupal/Core/Render/Renderer.php
@@ -524,7 +524,7 @@
       $elements['#markup'] = Markup::create($prefix . $elements['#children'] . $suffix);
     }
     else {
-      $elements['#markup'] = $elements['#children'];
+      $elements['#markup'] = Markup::create($elements['#children']);
     }
     // We've rendered this element (and its subtree!), now update the context.
     $context->update($elements);