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" ....

OnSelectedIndexChanged="UserList_SelectedIndexChan ged" Font-Size="10"

DataKeyNames="UserId" AllowPaging="true" PageSize="5"

AutoGenerateSelectButton="true">

<Columns>

<asp:BoundField DataField="UserName" HeaderText="User" />

<asp:BoundField DataField="UserId" Visible="false" />

....
<asp:UpdatePanel ID="UpdatePanel1" runat="server">

<contenttemplate>

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

</asp:Panel>

</contenttemplate>

<triggers>

<asp:AsyncPostBackTrigger ControlID="UserList"
EventName="SelectedIndexChanged" />

</triggers>