diff --git a/core/modules/system/src/Tests/Theme/ThemeTest.php b/core/modules/system/src/Tests/Theme/ThemeTest.php
index 5503fea..7c1e83a 100644
--- a/core/modules/system/src/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/src/Tests/Theme/ThemeTest.php
@@ -13,6 +13,7 @@
 use Symfony\Cmf\Component\Routing\RouteObjectInterface;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\Routing\Route;
+use Drupal\Component\Utility\SafeStringInterface;
 
 /**
  * Tests low-level theme functions.
@@ -65,7 +66,7 @@ function testThemeDataTypes() {
     $foos = array('null' => NULL, 'false' => FALSE, 'integer' => 1, 'string' => 'foo');
     foreach ($foos as $type => $example) {
       $output = \Drupal::theme()->render('theme_test_foo', array('foo' => $example));
-      if (is_string($output)) {
+      if ($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)));