VQMOD Error

I'm getting this error: 2015-10-02 19:59:31 - PHP Notice: Undefined variable: product_id in /home3/fwngdxze/public_html/store/vqmod/vqcache/vq2-catalog_controller_product_product.php on line 701

Here are the lines:

700 if (isset($this->error['keyword'])) {
701 $data['error_keyword'] = $this->error['keyword'];

I'm trying to upload audio using this extension: http://www.opencart.com/index.php?route ... a%20player

Noone would know, exept for those, buying such a Mod. You should therefore contact the Mod-Creator.
Just don't forget to give 'em Infos on Version, Theme, and Mod's used, otherways, he won't be able to assist either.
Just like here, without this important Information, you won't get ANY assistance.
Ernie