weird problem setting config indexes

[eluser]stormbytes[/eluser]
I'm autoloading a custom config file (app/config/arrow.php) with the following index:

Code:

$config['login_validation_url'] = site_url().'login/authorize';

For some reason I get an error when loading the application:

Code:

Fatal error: Call to undefined function site_url()

I'm autoloading the url helper as well.