diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index 2596212..ad82dd5 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -27,6 +27,7 @@
 use Drupal\Core\StreamWrapper\PublicStream;
 use Drupal\Core\Datetime\DrupalDateTime;
 use Drupal\block\Entity\Block;
+use Drupal\user\Plugin\Validation\Constraint\UserNameConstraintValidator;
 use Symfony\Component\HttpFoundation\Request;
 use Drupal\user\Entity\Role;
 
@@ -527,7 +528,7 @@ protected function drupalCreateUser(array $permissions = array(), $name = NULL)
 
     // Create a user assigned to that role.
     $edit = array();
-    $edit['name']   = !empty($name) ? $name : $this->getRandomGenerator()->string(8, TRUE, array('\Drupal\user\Plugin\Validation\Constraint\UserNameConstraintValidator', 'validCharacters'));
+    $edit['name']   = !empty($name) ? $name : $this->getRandomGenerator()->string(8, TRUE, array($this, 'randomUsernameValidate'));
     $edit['mail']   = $edit['name'] . '@example.com';
     $edit['pass']   = user_password();
     $edit['status'] = 1;
@@ -2701,4 +2702,19 @@ protected function prepareRequestForGenerator($clean_urls = TRUE, $override_serv
 
     return $request;
   }
+
+  /**
+   * Callback for random username validation.
+   *
+   * @see \Drupal\Component\Utility\Random::string()
+   *
+   * @param string $string
+   *   The random string to validate.
+   *
+   * @return bool
+   *   TRUE if the random string is valid, FALSE if not.
+   */
+  public function randomUsernameValidate($string) {
+    return $this->randomStringValidate($string) && !UserNameConstraintValidator::hasIllegalCharacters($string);
+  }
 }
diff --git a/core/modules/user/src/Plugin/Validation/Constraint/UserNameConstraintValidator.php b/core/modules/user/src/Plugin/Validation/Constraint/UserNameConstraintValidator.php
index 0d99486..38f0bbf 100644
--- a/core/modules/user/src/Plugin/Validation/Constraint/UserNameConstraintValidator.php
+++ b/core/modules/user/src/Plugin/Validation/Constraint/UserNameConstraintValidator.php
@@ -65,17 +65,4 @@ public static function hasIllegalCharacters($name) {
       $name);
   }
 
-  /**
-   * Checks if the username has only valid characters.
-   *
-   * @param string $name
-   *   The username to check.
-   *
-   * @return bool
-   *   TRUE if the username has only valid characters, FALSE if not.
-   */
-  public static function validCharacters($name) {
-    return !static::hasIllegalCharacters($name);
-  }
-
 }