.NET-MVC Modelbinder not binding because of primary key, all other fields are right. What's happening?

Question

I want to perform an Add object scenario in .NET using the modelbinder.

Function AddObject() As ActionResult
    Return View()
End Function
<AcceptVerbs(HttpVerbs.Post)> _
Function AddObject(<Bind(Exclude:="ObjectId")> ByVal o As BLL.Object) As ActionResult
    o.Save()
End Function

However, I get an error:

'o.ObjectId' is not declared or the module containing it is not loaded in the debugging session.

While inspecting the object, all other fields are set properly. I've tried all combinations regarding:

with/without Html.Hidden("ObjectId") to with/without <Bind(Exclude:="ObjectId")> and even setting o.ObjectId to Nothing, all with the same results.

What am I doing wrong?