combobox SelectedValueChanged มันขึ้น error ตอน pageload

private void cbbCustName_SelectedValueChanged(object sender, EventArgs e) {
			SqlConnection db = new SqlConnection(conn);

			try {
				db.Open();

				string sql2;

				sql2 = "select Cust_ID, Cust_Lastname from T_Customer where Cust_ID = '" + cbbCustName.SelectedValue + "'";
				SqlDataAdapter da2 = new SqlDataAdapter(sql2, db);
				DataSet ds2 = new DataSet();
				da2.Fill(ds2, "Cust");

				cbbLastname.DataSource = ds2;
				cbbLastname.DisplayMember = "Cust.Cust_Lastname";
			}
			catch (Exception ex) {
				MessageBox.Show(ex.Message);
			}
			finally {
				db.Close();
			}
		}