items_wrap ignoring custom values

I’m trying to add a class to the ul for the primary nav menu. I have this in my functions file:

function smpl_wp_nav_menu_args( $args = '' )
{
	$args['container'] = false;
	$args['theme_location'] = 'primary';
	$args['menu_class'] = 'menu clearfix';
	$args['items_wrap'] = '<ul class="%2$s">%3$s</ul>';
	echo '<pre>'; print_r($args); echo '</pre>';
	return $args;
}
add_filter( 'wp_nav_menu_args', 'smpl_wp_nav_menu_args' );

If I understand correctly, items_wrap is to get its “2” value from menu_class. I’ve also tried just putting the class directly into items_wrap. I even tried doing a simple change to an OL without any other arguments and it is still ignored. Help please?