diff --git a/core/modules/system/src/Tests/Theme/ThemeTest.php b/core/modules/system/src/Tests/Theme/ThemeTest.php
index c2c0e0b..5503fea 100644
--- a/core/modules/system/src/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/src/Tests/Theme/ThemeTest.php
@@ -60,11 +60,16 @@ function testAttributeMerging() {
    */
   function testThemeDataTypes() {
     // theme_test_false is an implemented theme hook so \Drupal::theme() service should
-    // return a string, even though the theme function itself can return anything.
+    // return a string or an object that implements SafeStringInterface, even though the
+    // theme function itself can return anything.
     $foos = array('null' => NULL, 'false' => FALSE, 'integer' => 1, 'string' => 'foo');
     foreach ($foos as $type => $example) {
       $output = \Drupal::theme()->render('theme_test_foo', array('foo' => $example));
-      $this->assertTrue(is_string($output), format_string('\Drupal::theme() returns a string for data type !type.', array('!type' => $type)));
+      if (is_string($output)) {
+        $this->assertTrue(TRUE, format_string('\Drupal::theme() returns a string for data type !type.', array('!type' => $type)));
+      } else {
+        $this->assertTrue($output instanceof SafeStringInterface, format_string('\Drupal::theme() returns an object that implements SafeStringInterface for data type !type.', array('!type' => $type)));
+      }
     }
 
     // suggestionnotimplemented is not an implemented theme hook so \Drupal::theme() service