@willbrowning I am also getting this error. Are you still facing this issue?
Hello,
What about this small snippet? it works fine.. i made it. it is listing only 2nd level of menu.
$menu = wp_nav_menu( array( 'menu_id' => 'nav-2', 'theme_location' => 'primary', 'echo'=>0 ) );
$menu = preg_match_all('/(<ul class="sub-menu">.*<\/ul>)(.*<\/li>)/s',$menu,$matches);
$menu = $matches[1][0];
$menu = str_replace('class="sub-menu"','id="nav-2"',$menu);
echo $menu;