Route into a controller sub-directory

[eluser]Edemilson Lima[/eluser]
I am having a problem with a routing...

At my routes.php, I have:

Code:

$route['([a-z]+)/([0-9]+)'] = "$1/details/index/$2";

If I go to: http://www.site.com/products/details/index/3
It works fine.
But, if I do it by the route: http://www.site.com/products/3
It calls products/details/index, but does not send the id code:

Code:

A PHP Error was encountered
Severity: Warning
Message: Missing argument 1 for Details::index()
Filename: products/details.php
Line Number: 11

"products" is a folder, "details" is the controller and "index" is its default function.