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

vendor/symfony/validator/Constraints/IsTrue.php

@@ -21,5 +21,11 @@ use Symfony\Component\Validator\Constraint;
  */
 class IsTrue extends Constraint
 {
+    const NOT_TRUE_ERROR = '2beabf1c-54c0-4882-a928-05249b26e23b';
+
+    protected static $errorNames = array(
+        self::NOT_TRUE_ERROR => 'NOT_TRUE_ERROR',
+    );
+
     public $message = 'This value should be true.';
 }