how to keep combo box as null before selecting the record , now always showing the first record in the combox item, without selection the record.

Even if I am not selecting  the item , In my combo box , always showing the  first record in the  in the combo box text. How can i keep it is null before selecting the record from ddwn. following is my code

 cmbCustomer.Items.Clear();
            kmInvoice objKmInvoice = new kmInvoice();
            DataTable t = new DataTable();
            t = objKmInvoice.CustomerList();
            cmbCustomer.DataSource = t;
            cmbCustomer.DisplayMember = "CustName";
            cmbCustomer.ValueMember = "Code";

Kind Regards

pol

polachan