Changed the ValidationError call to work the same way as before.

lib/instance-validator.js

@@ -150,7 +150,7 @@ InstanceValidator.prototype.validate = function() {
     self._customValidators()
   ]).then(function() {
     if (self.errors.length) {
-      return new sequelizeError.ValidationError('Validation error', self.errors);
+      return new sequelizeError.ValidationError(null, self.errors);
     }
 
     return new Promise(function(resolve) {