Here's a good article that explains that error message:http://msdn2.microsoft.com/en-us/library/1t3y8s4s(vs.80).aspx
It basically means that you're trying to pull the .Value out of a Nullable generic object, and the object doesn't have a value (it's NULL), so you can't pull the value. The article recommends using the GetValueOrDefault() method on the Nullable generic object.