Forms Data Controls :: Gridview And Ddl Error With Modal Extender?

on my page is a dropdown list and gridview. The gridview is bound to bound to a database using an object data source and has a select parameter (int)which is determined by the item selected in the ddl by the user.

it was working fine but now i have put the ddl in an update panel as i want to avoid the posback. The problem is now the pop up displays but the gridview does not show any data from the database. If I close the window and then try again it does but the first selection. i assume on the first attempt it is not getting the parameter.

<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" onselectedindexchanged="DropDownList1_SelectedIndexChanged" > <asp:ListItem Value="-1">select Lists</asp:ListItem>
<asp:ListItem Value="1">List1</asp:ListItem>
<asp:ListItem Value="2">List2</asp:ListItem>
<asp:ListItem Value="3">List3</asp:ListItem>
</asp:DropDownList>
</ContentTemplate>
</asp:UpdatePanel>
protected
void DropDownList1_SelectedIndexChanged(object sender,EventArgse)