public function addToWishlistBySerial($serial, $advance = '') {
        $item = unserialize($serial);
        if (!is_array($item) || empty($item)) {
            return $this->master->estore->getErrorResponse(Store::UNSERIALIZATION_ERROR, [$serial]);
        }

        $ids = array_keys($item);
        if (empty($ids)) {
            return $this->master->estore->getErrorResponse(Store::SERIALIZED_ITEM_IS_MALFORMED);
        }

        $post_id = (int)$ids[0];
        $skus = array_keys($item[$post_id]);
        if (empty($skus)) {
            return $this->master->estore->getErrorResponse(Store::SERIALIZED_ITEM_IS_MALFORMED);
        }

        $sku = (string)$skus[0];

        $exists = $this->guards->itemExists($post_id, $sku);
        if ($exists instanceof GenericError) {
            return $exists;
        }

        // check if logged in after all other checks pass so that we can add the item
        // to the wishlist automatically after a successful login
        $isloggedin = $this->guards->isLoggedIn();
        if ($isloggedin instanceof GenericError) {
            $isloggedin->setData($serial);
            return $isloggedin;
        }

        $res = CRUD::executeAddToWishlist($post_id, $sku, $serial, $advance);
        if ($res === false) {
            return $this->master->estore->getErrorResponse(Store::INTERNAL_SERVER_ERROR);
        }

        return true;
    }