I got an error, parent provider doesn't have boot method

[demonstan]

FatalThrowableError in MonobulletServiceProvider.php line 18:
Call to undefined method Illuminate\Support\ServiceProvider::boot()

    public function boot()
    {
        parent::boot();//not exist???

        if (in_array(config('app.env'), (array) config('services.monobullet.env', config('app.env')))) {
            $monolog = Log::getMonolog();
            $monolog->pushHandler(new PushbulletHandler(
                config('services.monobullet.token'),
                config('services.monobullet.recipients'),
                config('services.monobullet.level', Logger::INFO),
                config('services.monobullet.propagate', true)
            ));
        }
    }