وقتی برنامه رو اجرا میکنم به combotechernames.ValueMember = "teacherid" این error رو میده

Cannot bind to the new display member.
Parameter name: newDisplayMember

رویداد dropdown کامبو باکس

 private void comboteacher_DropDown(object sender, EventArgs e)

        {

            classes.teachercourse teachercourse = new classes.teachercourse();

           

            System.Data.DataSet dataset = teachercourse.fillcombotch();

            combotechernames.DataSource = dataset;

            combotechernames.DataSource = dataset;

            combotechernames.DisplayMember = "lname";

            combotechernames.ValueMember = "teacherid";





        }





اینم متود fill تو کلاس

 public System.Data.DataSet fillcombotch()

        {

            System.Data.DataSet dataset = new System.Data.DataSet();

            using (SqlConnection connection = new SqlConnection("Data Source=(local);Initial Catalog= ASATIR;Integrated Security=True"))

            {

                SqlDataAdapter myadapter = new SqlDataAdapter();



                SqlCommand command = new SqlCommand("select teacherid , lname from teacher", connection);

               



                

                //--open connection

                if (connection.State != System.Data.ConnectionState.Open)

                {

                    connection.Open();

                }



                myadapter.SelectCommand = command;



                myadapter.Fill(dataset, "teacher");





                if (connection.State != System.Data.ConnectionState.Closed)

                {

                    connection.Close();

                }

                if (command != null)

                {

                    command.Dispose();

                }





            }

            return dataset;

        }