Issue with qmt_base_url filter in 3.8.1

Hi,

After updating to WP 3.8.1 the taxonomy filter does not seem to work anymore when using this function to customize the base url:

add_filter( 'qmt_base_url', 'my_qmt_base_url' );
function my_qmt_base_url( $base ) {
     if( get_post_type() == 'training') {
         return home_url().'/trainingen/';
    }

    if( get_post_type() == 'adviseur') {
        return home_url().'/adviseurs/';
    }
}

When adding that function I also get the warning:

rtrim() expects parameter 1 to be string in /wp-includes/formatting.php on line 1497

Any ideas?

Thanks in advance!

http://wordpress.org/plugins/query-multiple-taxonomies/