adding edit button to datagrid problem

[slinky]

I had a successfully deployed datagrid reading an XML file and
showing the data:

Private Function MakeDataView() as DataView
Dim myDataSet As New DataSet()
myDataSet.ReadXml(Server.MapPath("TimberSales.xml" ))
Dim view As DataView = New DataView(myDataSet.Tables(0))
view.AllowDelete = False
view.AllowEdit = True
view.AllowNew = False
view.Sort = "Year ASC"
Return view
End Function
Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs)
Dim view as DataView = MakeDataView()
dgTimber.DataSource = view
dgTimber.AllowSorting = True
dgTimber.DataBind()
End Sub
__________________________________________________ ____________________

I wanted to add an "Edit" button as desribed at:
http://msdn.microsoft.com/en-us/libr...8(VS.71).aspx:

Private Sub dgTimber_EditCommand(ByVal source As Object, _
ByVal e As System.Web.UI.WebControls.DataGridCommandEventArgs ) _
Handles dgTimber.EditCommand
dgTimber.EditItemIndex = e.Item.ItemIndex
dgTimber.DataBind()
End Sub

Dim newData As String
Dim aTextBox As TextBox
aTextBox = CType(e.Item.Cells(1).Controls(0), TextBox)
**** says "aTextBox" is not declared, (?)
newData =
aTextBox.Text **** says
"newData" is not declared, (?)

Private Sub dgTimber_UpdateCommand(ByVal source As Object, _
ByVal e As System.Web.UI.WebControls.DataGridCommandEventArgs ) _
Handles dgTimber.UpdateCommand
Dim quantityCell As TableCell = e.Item.Cells(6)
Dim quantityBox As TextBox = _
CType(quantityCell.Controls(0), TextBox)
Dim quantity As Integer = System.Int32.Parse(quantityBox.Text)
dgTimber.EditItemIndex = -1
dgTimber.DataBind()
End Sub
When I hit the Edit Button I get this:

Invalid postback or callback argument. Event validation is enabled
using <pages enableEventValidation="true"/>
in configuration or <%@ Page EnableEventValidation="true" %in a
page. For security purposes, this feature ....etc.

Does anything obvious stabd out as the potential probelm? Thank You!