The member with identity ' ' does not exist in the metadata collection.\r\nParameter name: identity

I simplified the code a little while trying to debug:

[HttpPost]
    public ActionResult Register(User model)
    {
        DateTime bla = new DateTime(2012, 12, 12);
        try
        {
            User user = new User
            {
                gid = 1,
                cid = 1,
                firstName = model.firstName,
                lastName = model.lastName,
                email = model.email,
                username = model.username,
                password = model.password,
                creationDate = bla,
                active = 1
            };
            myContext.Users.AddObject(user);
            myContext.SaveChanges();

        }
        catch (Exception ex)
        {
            throw ex;
        }

        return View();               
    }

The values are transmited accordingly. Users table:

[id] [int] IDENTITY(1,1) NOT NULL,
[cid] [int] NULL,
[gid] [int] NULL,
[firstName] [nvarchar](100) NOT NULL,
[lastName] [nvarchar](100) NOT NULL,
[email] [nvarchar](max) NOT NULL,
[username] [nvarchar](100) NOT NULL,
[password] [nvarchar](100) NOT NULL,
[creationDate] [datetime] NOT NULL,
[active] [int] NOT NULL,

CONSTRAINT [PK_Users_3213E83F0AD2A005] PRIMARY KEY CLUSTERED

I deleted all the foreign keys to be sure that nothing affects it. I am qute certain that at a previous moment it was working, but now I can not figure where the issue is. It crashes while performing the savechanges:

{"An error occurred while updating the entries. See the inner exception for details."}
{"The member with identity '' does not exist in the metadata collection.\r\nParameter name: identity"}

The issue was reproducing because of a trigger that was on the users table. Removed it and the issue is not reproducing anymore.