diff -u b/core/tests/Drupal/Tests/Component/Utility/RandomTest.php b/core/tests/Drupal/Tests/Component/Utility/RandomTest.php
--- b/core/tests/Drupal/Tests/Component/Utility/RandomTest.php
+++ b/core/tests/Drupal/Tests/Component/Utility/RandomTest.php
@@ -8,6 +8,7 @@
 namespace Drupal\Tests\Component\Utility;
 
 use Drupal\Component\Utility\Random;
+use Drupal\Component\Utility\String;
 use Drupal\Tests\UnitTestCase;
 
 /**
@@ -42,7 +43,7 @@
     try {
       for ($i = 0; $i <= 100; $i++) {
         $str = Random::string(1);
-        $this->assertFalse(isset($names[$str]), '');
+        $this->assertFalse(isset($names[$str]), String::format('Generated duplicate random string !string', array('!string' => $str)));
         $names[$str] = TRUE;
       }
     }
@@ -69,7 +70,7 @@
     try {
       for ($i = 0; $i <= 100; $i++) {
         $str = Random::name(1);
-        $this->assertFalse(isset($names[$str]), '');
+        $this->assertFalse(isset($names[$str]), String::format('Generated duplicate random name !name', array('!name' => $str)));
         $names[$str] = TRUE;
       }
     }