ado.net Concurrency violation

Question

My first time using ADO.net. Trying to make database of Users.

First I populate my DataSet:

adapter.AcceptChangesDuringFill = true;
adapter.AcceptChangesDuringUpdate = true;
adapter.Fill(dataset);

To create a user:

User user = new User();

user.datarow = dataset.Users.NewUsersRow();
user.Name = username;
user.PasswordHash = GetHash(password);
user.Rights = UserRights.None;
users.Add(user);

dataset.Users.AddUsersRow(user.datarow);
adapter.Update(dataset);

When a user property is modified:

adapter.Update(dataset);

Creation by itself is fine. If I take an existing user and make multiple changes, fine. Multiple creations in a row, fine. Creation followed by a property change, I get this: "Concurrency violation: the UpdateCommand affected 0 of the expected 1 records."

Any ideas?