protected function getActiveUser()
    {
        if (!Config::has('culpa.users.active_user')) {
            return Auth::check() ? Auth::user() : null;
        }

        $fn = Config::get('culpa.users.active_user');
        if (!is_callable($fn)) {
            throw new \Exception('culpa.users.active_user should be a closure');
        }

        return $fn();
    }