error after upgrade

apologies if this is in the wrong place
i have just upgraded vqmod but i am now getting this error
Undefined index: cost in /public_html/vqmod/vqcache/vq2-system_library_cart.php on line 2

line 222 says
'cost' => $product_query->row['cost'],

any help on this would be appreciated

forgot to say but this only happens when items are added to the cart

anyone