public static function validateComboSetOnAddToWishlist($null, $args) {
        // we need to make sure the item is a combo-set item
        $comboset = API\ComboSet::getComboSetFromSkuCode($args['post_id'], urldecode($args['sku']));
        if ($comboset instanceof GenericError) {
            // error fetching automatically sets the response code (bad design...)
            http_response_code(200);
            return $null;
        }

        $pvals = API\Cart::getAddToCartPostValues($comboset->getId());
        $groupids = $pvals['groupids'];

        // check that all group IDs and item IDs exist. Check that required groups have selections
        $error = API\PurchaseSanityChecks::comboSetSelectionsAreValid($comboset->getId(), $groupids);
        if ($error instanceof GenericError) {
            return $error;
        }

        return $null;
    }