diff --git a/core/lib/Drupal/Core/Template/TwigExtension.php b/core/lib/Drupal/Core/Template/TwigExtension.php
index 15ec455066..7d67ff991c 100644
--- a/core/lib/Drupal/Core/Template/TwigExtension.php
+++ b/core/lib/Drupal/Core/Template/TwigExtension.php
@@ -557,7 +557,7 @@ public function renderVar($arg) {
     }
 
     // Optimize for scalars as it is likely they come from the escape filter.
-    if (is_scalar($arg) || $arg instanceof MarkupInterface || $arg instanceof \Twig_Markup) {
+    if (is_scalar($arg)) {
       return $arg;
     }
 
diff --git a/core/modules/system/src/Tests/Theme/TwigExtensionTest.php b/core/modules/system/src/Tests/Theme/TwigExtensionTest.php
index b2536fa531..4e93eb9251 100644
--- a/core/modules/system/src/Tests/Theme/TwigExtensionTest.php
+++ b/core/modules/system/src/Tests/Theme/TwigExtensionTest.php
@@ -94,7 +94,7 @@ public function testRenderMarkup() {
     /** @var \Drupal\Core\Template\TwigExtension $extension */
     $extension = \Drupal::service('twig.extension');
     $markup = new FormattableMarkup('<span>This is a @markup</span>', ['@markup' => 'MarkupInterface']);
-    $this->assertIdentical($extension->renderVar($markup), $markup, 'TwigExtension::renderVar() returns MarkupInterface correctly.');
+    $this->assertIdentical($extension->renderVar($markup), '<span>This is a MarkupInterface</span>', 'TwigExtension::renderVar() returns MarkupInterface correctly.');
   }
 
 }
diff --git a/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module b/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module
index 8a613c31a0..169f0d4c98 100644
--- a/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module
+++ b/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module
@@ -27,9 +27,9 @@ function twig_theme_test_theme($existing, $type, $theme, $path) {
   $items['twig_theme_test_trans_render_array'] = array(
     'variables' => array(
       'var' => array(
-        '#prefix' => '<b>',
+        '#prefix' => '<strong>',
         '#markup' => 'trans render array',
-        '#suffix' => '</b>',
+        '#suffix' => '</strong>',
       ),
     ),
     'template' => 'twig_theme_test.trans_render_array',