I followed this article on making a table-per-type inheritance model for my entities, but I get the following error when I try to add an instance of a subclass to the database.

Here is how I create the subtype:

var cust = db.Users.CreateObject<Customer>(); // Customer inherits User
db.Users.AddObject(cust);
db.SaveChanges();

I get the following error when I make that last call:

"A value shared across entities or associations is generated in more than one location. Check that mapping does not split an EntityKey to multiple store-generated columns."

With the following inner exception:

"An item with the same key has already been added."

Any ideas on what I could be missing?