Datagridview giving errors on adding a new row on the comboboxcell

Hello,

I have problems with a comboboxcolumn in a datagridview: As I want to insert a new row, I get a "System.ArgumentException: The value of the DataGridViewComboBoxCell is invalid." I am using Linq to SQL to populate the data from different tables:

- Table Customer (ID (key), Name,...)
- Table Address (ID (key), CustomerID, Street,...)
- Table ContactPerson (ID (key), AddressID, Name, Title,...)

My Linq query to populate the ContactDataGridView(which is bound to the TextBox RelationIdTextBox), is following:

Code:

---

    var y = from r in BH.Relations
                        where r.RelationID.ToString() == relationIDTextBox.Text
                        join v in BH.Addresss on r.RelationID equals v.RelationID
                        join c in BH.ContactPersons on v.AddressID equals c.AddressID
                        select c;
    contactPersonsBindingSource.DataSource = y;
    ContactDataGridView.DataSource = contactPersonsBindingSource;

---

This works fine.

In this ContactDataGridView I have a ComboBoxCell AddressID which populates the streetnames instead of the AddressID, using following linq query:

Code:

---

    var addr = from a in BH.Addresss
                          where a.RelationID.ToString() == relationIDTextBox.Text
                          select new { AddressID = a.AddressID, Street = a.Street };
    ContactAddressBindingSource = addr;

---

This works fine as well.

That is, untill I want to add a new row in this datagridview. Then I get the error "System.ArgumentException: The value of the DataGridViewComboBoxCell is invalid."

Can anyone help to make me understand what I am doing wrong? I really cannot figure it out!

Thanks!

I tried it with other tables within my Database, and I keep getting the same errors. I noticed that I only get this error when the comboboxcell may not contain null values. Now, how can I prevent this error and let the user decide which value he wants to select from within the possible values in this combobox?