custom_post_type with umlaut in name

i now changed the navigation function to check for the url, since this one was right in all cases (i do have some other custom_post_types, that worked fine and still do).
i just needed to strip the slashes from the url:

add_filter('nav_menu_css_class', 'current_type_nav_class', 10, 2 );
function current_type_nav_class($classes, $item) {
    $post_type = get_post_type();
    if ($item->post_name != '' && str_replace("https://wordpress.stackexchange.com/", "", $item->url) == $post_type) {
        array_push($classes, 'current-menu-item');
    };
    return $classes;
}