Issue when using custom_url_rewrite

For a specific setup i'm using custom_url_rewrite_outbound to alter links to add a query string (f.e every link now has ?somevar=x).
Admin menu uses url() to get the base path, but this generates an incorrect link.

    // The base path to use for cache requests depends on whether clean URLs
    // are enabled, whether Drupal runs in a sub-directory, and on the language
    // system configuration. url() already provides us the proper path, but we
    // additionally need to ensure that it ends with a slash.
    $settings['basePath'] = rtrim(url(''), '/') . '/';

In my case this generates a cache link to f.e.

http://mydomain/?somevar=x/js/admin_menu/cache/cd1537ffda2f98ac7b4110bb4...

because my alterations are present in an url() call.

It works when I change it to

$settings['basePath'] = rtrim(base_path(), '/') . '/';