diff -u b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
--- b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
@@ -71,9 +71,11 @@
   }
 
   /**
-   * Tests that Twig templates handle PHP various data types correctly.
+   * Tests that the Twig engine handles PHP data correctly.
    */
   function testTwigVariableDataTypes() {
+    // Test NULL values.
+    // @todo Test multiple PHP data types.
     config('system.theme')
       ->set('default', 'test_theme_twig')
       ->save();
diff -u b/core/modules/system/tests/themes/test_theme_twig/test_theme_twig.theme b/core/modules/system/tests/themes/test_theme_twig/test_theme_twig.theme
--- b/core/modules/system/tests/themes/test_theme_twig/test_theme_twig.theme
+++ b/core/modules/system/tests/themes/test_theme_twig/test_theme_twig.theme
@@ -8,8 +8,7 @@
  * Implements THEME_preprocess_theme_test_template_test().
  */
 function test_theme_twig_preprocess_theme_test_template_test(&$variables) {
-  // Build an array of type => value pairs for testing various data types.
-  // @todo: Coverage for all PHP data types.
+  // Build an array of type => value pairs for testTwigVariableDataTypes().
   $types = array(
     'null' => NULL,
   );