Undefined Error on Thank You Page

Since installing your software, I am now getting the below undefined error on my thank you page. Could you tell me what I need to do to fix this. My site is going live today so a quick response would be appreciated. Thanks.

Notice: Undefined property: ps_payment::$payment_code in /clientdata/zeus-dynamic-1/s/h/shamrockshirts.com.au/www/2011website/administrator/components/com_virtuemart/classes/ps_checkout.php on line 1237

I think you should open the file and look at the line being referred to. Not sure what the problem is, do not recognize the error.