Class based on nav item title not working

Hi,

I am coding a custom theme. I’d like to add a special class to the
<li> tags based on each’s title but the $items variable/parameter for nav_menu_css_class returns empty (var_dump($items) returns NULL).

Hence ‘title_’ is all that’s returned in the following function:

function filter_nav_item_css_attributes($classes, $item)
{
    if(is_array($classes)){
        $classes = array_intersect($classes, array('current-menu-item'));
        array_push($classes,'title_'.$item->title);
        return $classes;
    } else {
        return ' ';
    }
}