Asp.net - How To Add A Row To A Data-bound Gridview Without Causing Viewstate Errors

I'm trying to add a row grouping row to my data-bound gridview. It works fine on the first response, but at the postback i got the "Failed to load viewstate" error.There is the code for the GridView's RowDataBound event:

Private Sub AddGroupingRow(ByRef eRow As GridViewRow, ByVal Css As String, ByVal ColSpan As Integer, ByVal Txt As String)
Dim cell As New TableCell()
cell.ColumnSpan = ColSpan
cell.CssClass = "Spacing FieldCell"
cell.Text = Txt
Dim row As New GridViewRow(-1, -1, DataControlRowType.DataRow, DataControlRowState.Normal)
row.CssClass = Css
row.Cells.Add(cell)
[code]...