Type casting, where some chunk of data (eg. "1" as a string) is trying to be cast to an integer somewhere. The only thing you could really do here is fire up Visual Studio and hit the page while debugging. Once you've done that should only take a few minutes to find out which bit of code is causing the error...just step through until it pukes. It's an error we've all seen before at some point or another when working with SQL...but not something we've ever encountered on this page or with this control, and without having access to the code it's all but impossible to track down within the context you've provided here.
<a href="http://www.aspdotnetstorefront.com">Shopping Cart Software</a>