default_controller not working as I expected

Hi, I'm having some trouble understanding a problem with my routes.php config. The current working route

PHP Code:

$route['default_controller'] = 'welcome'; 


works fine, but when I change it to

PHP Code:

$route['default_controller'] = 'admin/user/login'; 


I get a 404. if I go to my virtual hosts http://cms.dev/admin/user/login though, the page is displayed as expected. What's happening?

Thanks.