What is Error-The multi-part identifier "System.Data.DataRowView" could not be bound. in code bellow-

Question

what is error
Error-The multi-part identifier "System.Data.DataRowView" could not be bound.
in code bellow-
 con = new SqlConnection(@"Data Source=COMP12\SQLEXPRESS;Initial Catalog=TNTDB;Integrated Security=True");
  con.Open();
  string str = "select Material_Description from InwardRegister where PO=" + txt_po_no.SelectedValue + "";
  SqlCommand cmd = new SqlCommand(str, con);
  dr = cmd.ExecuteReader();
  while (dr.Read())
  {

  cmb_mat_name.Items.Add(dr["Material_Description"]);

  }
  dr.Close();
  cmb_mat_name.Text = "-Select-";
  con.Close();