any ajax person to help or grid view

[kal]

I have a gridview on my page - the 'onSelIndexChan ' of which is wired to a
ajax update panel - via triggers.

this works fine but the grids paging wont work - or rather it fires a post
back but the next page isnt displayed.
and then when i click the page link again it gives an invalid argument
error.

There is no code in the event handlers

If i take out the ajax update panel the paging works again!

Here are some snippets

<asp:GridView ID="UserList" runat="server" ....

OnSelectedIndex Changed="UserLi st_SelectedInde xChanged" Font-Size="10"

DataKeyNames="U serId" AllowPaging="tr ue" PageSize="5"

AutoGenerateSel ectButton="true ">

<Columns>

<asp:BoundFie ld DataField="User Name" HeaderText="Use r" />

<asp:BoundFie ld DataField="User Id" Visible="false" />

....
<asp:UpdatePane l ID="UpdatePanel 1" runat="server">

<contenttemplat e>

<asp:Panel ID="UserDetails Panel" runat="server" ...

</asp:Panel>

</contenttemplate >

<triggers>

<asp:AsyncPostB ackTrigger ControlID="User List"
EventName="Sele ctedIndexChange d" />

</triggers>