Set TargetControlID For ModalPopupExtender As A Button In A DataGrid Header?

I am trying to add a ModalPopup to a GridView, by adding a button in the data grid header:

<asp:GridView ID="GridViewTradeDetail" />
<Columns>
<asp:TemplateField HeaderText="B/S">
<HeaderTemplate>
<asp:Button ID="ButtonShowBuySellPopup" runat="server" Text="Set B/S" />
</HeaderTemplate>
<EditItemTemplate>
<asp:TextBox ID="TextBox1" runat="server" Text='<%# Bind("buy_sell") %>'></asp:TextBox>
</EditItemTemplate>
<ItemTemplate>
<asp:Label ID="Label1" runat="server" Text='<%# Bind("buy_sell") %>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
<asp:BoundField DataField="trade_date" HeaderText="TD" />
</Columns>
</asp:GridView>
...
<asp:Panel ID="PanelBuySell" runat="server">
... panel content ...
</asp:Panel>
<ajaxToolkit:ModalPopupExtender ID="ModalPopupExtenderBuySell" runat="server"
TargetControlID="ButtonShowBuySellPopup" PopupControlID="PanelBuySell" />
<asp:Button ID="ButtonShowBuySellPopup" runat="server" Text="Set B/S" />

But I get an error that the ModalPopupExtender cannot find the button ButtonShowBuySellPopup (If I move the button out of the grid header it's OK).

How can I link the popup to a button (or better still, text link) in the header of a Gridiew?