how to prevent redirect function adding index.php to urls

I'm using the CI function redirect in my web app. The problem I'm having is that if I redirect a user to /some/path/internally, then the redirect function inserts "index.php" into the url it redirects to. For example I created a controller Test:

Code:

class Test extends CI_Controller {
 public function index() {
   die(site_url("/some/path/internally"));
 }
}

The output is this:

Code:

http://example.com/index.php/some/path/internally

whereas I'd rather have it be:

Code:

http://example.com/some/path/internally

or even just

Code:

/some/path/internally

Is there some configuration setting I'm missing or something?

apache
mod_rewrite
.htaccess

application/config/config.php:
$config['index_page'] = "" // => remove "index.php"