ROW numbering format

[Kaizen]

i already created gridbound column.. wanted to add numbering column due to user request.
i get Error: system.data.datarowview at No column.

<telerik:GridBoundColumn UniqueName="RowNumber" HeaderText="No" EditFormColumnIndex="1"></telerik:GridBoundColumn>

aspx.cs:

        protected void RadGrid1_ItemDataBound(object sender, GridItemEventArgs e)
        {
            if (e.Item is GridDataItem)
            {
                GridDataItem dataItem = (GridDataItem)e.Item;
                dataItem["RowNumber"].Text = Convert.ToString(e.Item.ItemIndex + 1);


            }
        }