on saving records in data table getting error argumentexception was unhandled

private void cmdSave_Click(object sender, EventArgs e)
        {

            dt = hRDataSet.Tables["empdetails"];
            dr = dt.NewRow();
            


                dr[0] = txtccode.Text;    //argumentexception was unhandled
                dr[1] = txtEmployeeName.Text;
                dr[2] = txtAddress.Text;
                dr[3] = txtState.Text;
                dr[4] = txtCountry.Text;
                dr[5] = cbDepartment.SelectedItem;
                dr[6] = cbDesignation.SelectedItem;
                dt.Rows.Add(dr);
                empdetailsTableAdapter.Update(hRDataSet);
                txtccode.Text = System.Convert.ToString(dr[0]);
                txtccode.Enabled = false;
                txtEmployeeName.Enabled = false;
                txtAddress.Enabled = false;
                txtState.Enabled = false;
                txtCountry.Enabled = false;
                cbDepartment.Enabled = false;
                cbDesignation.Enabled = false;
                this.empdetailsTableAdapter.Fill(this.hRDataSet.empdetails);
                cmdAdd.Enabled = true;
                cmdSave.Enabled = false;
            }