Extending Auth in a Service Provider

AuthServiceProvider.php

<?php 

// I am trying to extend Auth using a custom driver for MongoDB in the package's service provider's register method:

$this->app['auth']->extend('mongo', function($app)
{
	return new Auth\MongoUserProvider($app['db']->connection('mongo'), $app['config']['auth.table']);
});

// But getting error that "Class auth does not exist in Container at line 485.". I tried doing this:

dd($this->app['auth']);

// But got the error again. I am sensing that the 'auth' is not bound yet to the app in the service provider.
// However, if I do the following in global.php file, it works just fine.

Auth::extend(...);

// Anything special about Auth component that doesn't allow it to extend in service provider?