add_query_arg not working

I added a filter to append a parameter onto the URL when navigating in categories. This is used to sort posts by their votes when browsing a category only if a sort parameter is set.

For instance when you click view all posts with most votes, posts with high votes are displayed. From there you can view most voted posts by category by appending a sort=most_voted or sort=doleast_voted to the URL with cat=?.

add_filter( 'category_link','append_parameter', 10, 2 );

function append_parameter( $link, $query ) {
$my_parameter = $query->query_vars['sort']; //get sort value
if ( isset($my_parameter) ) { //if browsing posts by votes
    $link = add_query_arg( 'sort', $my_parameter, $link );
}
    return $link;
}

I can’t figure out why the sort parameter isn’t appended to the URL. This works however without the if statements and a value instead of the $my_parameter in the add_query_arg.

EDIT: New working code

    add_filter( 'category_link','append_parameter', 10, 2 );

    function append_parameter( $link, $my_parameter ) {
    $my_parameter = $_GET['sort']; //get sort value       
    if ( isset($my_parameter) ) { 
        $link = add_query_arg( 'sort', $my_parameter, $link );
    }
        return $link;
    }