I get a "Object reference not set to an instance of an object." error when I run this simple VB form

Imports Oracle.DataAccess.Client
Imports Telerik.WinControls.UI

Public Class frmTreeView

Private cmdTemp As New OracleCommand
Private dvTemp As DataView
Private dtTemp As New DataTable
Private daTemp As OracleDataAdapter
Private dsTemp As New DataSet
Private Sub frmTreeView_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
  With cmdTemp
    .CommandType = CommandType.StoredProcedure
    .Connection = db.oraCn
    .CommandText = "badgernet.web_read_util.group_tree"
    .Parameters.Clear()
    .Parameters.Add("out_rs", OracleDbType.RefCursor, DBNull.Value, ParameterDirection.Output)
  End With
  daTemp = New OracleDataAdapter(cmdTemp)
  dtTemp.Clear()
  daTemp.Fill(dtTemp)
  dsTemp.Tables.Add(dtTemp)

  tvGroups.DataSource = dsTemp
  tvGroups.DisplayMember = "group_name"
  tvGroups.ValueMember = "object_id"
  tvGroups.ParentIDMember = "PARENT_ID"  ' ERRORS ON THIS LINE
  tvGroups.DataMember = "dtTemp"

End Sub
End Class

It runs if I comment out the offending line, without hierarchical support. There is a field named "parent_id". Any suggestions?
My data source looks like this.  I need the child field to be group_id, but the valuemember to be object_id, if that's possible.

object_id  group_id  parent_id   group_name
100 100 nullFootball
101 101 100 Offense
103 103 101 O-Line
110 110 101 Tight Ends
111 111 101 Recievers
112 112 101 Backs
65234 0 112 Duckworth, Jeff
113 113 112 Tailbacks
114 114 112 Fullbacks
102 102 100 Defense

Later
Art