Update to Drupal 8.1.0. For more information, see https://www.drupal.org/drupal-8.1.0-release-notes

vendor/symfony/validator/Constraints/IsFalse.php

@@ -21,5 +21,11 @@ use Symfony\Component\Validator\Constraint;
  */
 class IsFalse extends Constraint
 {
+    const NOT_FALSE_ERROR = 'd53a91b0-def3-426a-83d7-269da7ab4200';
+
+    protected static $errorNames = array(
+        self::NOT_FALSE_ERROR => 'NOT_FALSE_ERROR',
+    );
+
     public $message = 'This value should be false.';
 }