trouble with Gridview and DataKeyNamese attribute

I have a grid view that returns a image button in the first column and i need to get the value of the second column in the datagrid to pass that value to back to beable to edit the record.
I also set up a datakeyname using that same field name from the database.
here is my code:
   Protected Sub GridView2_RowEditing(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewEditEventArgs) Handles GridView2.RowEditing
        Dim billid As Integer

        billid = GridView2.SelectedRow.Cells(2).Text

        Response.Redirect("main.aspx?mode=edit&billid=" & billid)

    End Sub

I get  a null excetion on this line everytime

     billid = GridView2.SelectedRow.Cells(2).Text

what is the problem?

Please help
Thanks

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