Datagridviewcomboboxcolumn default equal to another datagridview column

See more:

C#

DataGridView

Nullable

I have added a dgvcomboboxcol to an existing dgv. The dgvcomboboxcol will replace an existing column and allow for user selection.

The code adds a list of items a user can select from, but I'd like to set the dgvcomboboxcol with a default the same as the column it replaced. The code below is copied from another question and compiles, but gives an error "dgvcomboboxcell value is not valid".

Thanks for any help.

What I have tried:

public void BOOK_COMBO2()
        {

            DataGridViewComboBoxCell cb_cell = new DataGridViewComboBoxCell();
            DataGridViewComboBoxColumn cb_col = new DataGridViewComboBoxColumn();

            // Contract field
            cb_col.Items.AddRange("AP-21-Apr-16", "HSI-28-Apr-16", "");
            cb_col.FlatStyle = FlatStyle.Flat;
            cb_col.HeaderText = "newCONTRACT";
            cb_col.Width = 50;
            cb_col.ValueType = typeof(string);

            grdBOOK.Columns.Insert(5, cb_col);

            foreach (DataGridViewRow item in grdBOOK.Rows)
            {
                item.Cells[5].Value = (string)item.Cells[3].Value;      
            }

        }