You'll have to back up through your code from there. It would be useless speculation to guess what's going on. I've seen similar error messages when say a form tries to save a record when you close it. That may be a starting point...
Ken, pardon me for jumping in, but if I might jump in really quick - There is a spot which seems to have this error more than most. It is when a DLookup returns a NULL. So, check for any DLookups and make sure you have used the NZ function with them to deal with nulls.