public function onTrial($subscription = 'default', $plan = null)
    {
        if (func_num_args() === 0 && $this->onGenericTrial()) {
            return true;
        }

        $subscription = $this->subscription($subscription);

        if (is_null($plan)) {
            return $subscription && $subscription->onTrial();
        }

        return $subscription && $subscription->onTrial() &&
               $subscription->stripe_plan === $plan;
    }