在NerdDinner中首次尝试Linq to Sql - 规则违规会阻止保存

I'm trying to go through the NerdDinner example chapter from the ASP.Net MVC 1.0 and I've come across an error. Everything was hunky dory until I got to the part where I need to edit a dinner. I've followed the guide word for word from the creation of the project until this point (at least the best I can tell). However, when I call the example