voce_cached_nav_menu return NULL

if trying to use voce_cached_nav_menu to replace wp_nav_menu in my code, I always get NULL value. So, I put return in voce_cached_nav_menu and it's working correctly.

This line in my code that I replace:

$html = '<' . $navtype . $navclass . '>' . wp_nav_menu( $wp_nav ) . '</' . $navtype . '>';

Then I replaced it to:

$html = '<' . $navtype . $navclass . '>' . voce_cached_nav_menu( $wp_nav ) . '</' . $navtype . '>';

and it's just working correctly to print out my nav menu.