The Symfony\Component\Validator\Constraints\File class has an __isset method on line 90 which calls an unexisting __isset method of its parent class Symfony\Component\Validator\Constraint , triggering the following error :

Attempted to call an undefined method named "__isset" of class "Symfony\Component\Validator\Constraint".

This is how the Symfony\Component\Validator\Constraints\File method line 90 looks like :

    public function __isset($option)
    {
        if ('maxSize' === $option) {
            return true;
        }

        return parent::__isset($option);
    }