I’m not sure where to post this but I am a bigger so I put it here
I installed zen cart for xoops on site (works great) I added wishlist plugin
When i go to add product to wishlist this is what I get


Fatal error: Call to undefined function un_is_empty() in /home/mysite/public_html/modules/zox/includes/modules/pages/wishlist/header_php.php on line 13


This is what header_php.php on server

if (!$_SESSION['customer_id']) {
$_SESSION['navigation']->set_snapshot();
zen_redirect(zen_href_link(FILENAME_LOGIN, '', 'SSL'));
}

// Get wishlist class and instantiate
require_once(DIR_WS_CLASSES . 'wishlist_class.php');
$oWishlist = new un_wishlist($_SESSION['customer_id']);

// Use specified wishlist if wid set, else use default wishlist
$id = isset($_REQUEST['wid']) ? (int) $_REQUEST['wid'] : '';
if ( ! un_is_empty($id) ) {
$oWishlist->setWishlistId($id);
if ( ! $oWishlist->hasPermission() ) {
zen_redirect(zen_href_link(UN_FILENAME_WISHLISTS, '', 'SSL'));
}
} else {
$id = $oWishlist->getDefaultWishlistId();
}

require(DIR_WS_MODULES . 'require_languages.php');
$breadcrumb->add(NAVBAR_TITLE);


any suggestions?