Laravel 5阶段服务器403 on / route

Even though I already have Route::get('/', 'WelcomeController@index'); inside routes.php, when I access myherokuappurl.heroku.com (this is not the real url) it shows a 403 error.

The WelcomeController is the scaffolded one by artisan, it contains:

public function __construct() {
    $this->middleware('guest');
}

public function index() {
    return view('welcome');
}

Any ideas? using laravel 5

Edit: it works fine on my Laravel/Homestead development environment

Update 1: these are my heroku web server setup

// Procfile
web: vendor/bin/heroku-php-nginx -C nginx.conf public/

// nginx.conf
location / {
    try_files $uri $uri/ /index.php?$query_string;
}

Update 2: The 403 only happens on / route, but on /myresource/index or /myresource/create works fine