Error while populating WebDatagrid: 'column' argument cannot be null.

hi all, it might be simple but i don't now what to do. I keep geting this error when i try to relate datasets. here is my code


            SqlConnection StrConn = new SqlConnection(ConfigurationSettings.AppSettings["StrConnectionString"]);
            SqlDataAdapter daUnassignedData = new SqlDataAdapter("select b.company, b.Address, b.City from DMD_Data.dbo.Brokers b", StrConn);
            SqlDataAdapter daUnassignedData1 = new SqlDataAdapter("select l.first_name, l.last_name,l.email_address from INTRANET.dbo.LO_INFORMATION l, DMD_Data.dbo.brokers b where b.brokers_id = l.broker_id", StrConn);
            daUnassignedData.SelectCommand.CommandType = CommandType.Text;
            daUnassignedData1.SelectCommand.CommandType = CommandType.Text;
            DataSet ds = new DataSet();
            daUnassignedData.Fill(ds, "brokers");
            daUnassignedData1.Fill(ds, "los");

            ds.Relations.Clear();
            ds.Relations.Add("rel", ds.Tables["brokers"].Columns["brokers_id"],
                ds.Tables["los"].Columns["broker_id"]);
            this.UltraWebGrid1.DataSource = ds.Tables["brokers"].DefaultView;
            this.UltraWebGrid1.DataBind();
ps. the broker_id in los dataset have null values

thanks