ASP.Net AJAX DropDownList + UpdatePanel

Hi,

I have 2 dropdownlists and I want SiteCombo to be updated asyncronously when RegionCombo's SelectedIndexChanged event is fired.

Heres my code

<div class="dropDownHeader">Region:<br /></div>
                <asp:DropDownList
                    id="RegionCombo"
                    runat="server"
                    OnSelectedIndexChanged="RegionCombo_SelectedIndexChanged"
                    cssclass="dropDownNav"
                    AutoPostBack="true">
                </asp:DropDownList>
                <asp:UpdatePanel id="regionPanel" runat="server" UpdateMode="Conditional">
                    <Triggers>
                        <asp:AsyncPostBackTrigger ControlID="RegionCombo" EventName="SelectedIndexChanged" />
                    </Triggers>
                    <ContentTemplate>
                        <div class="dropDownHeader">Site:<br /></div>
                        <asp:DropDownList
                            id="SiteCombo"
                            runat="server"
                            OnSelectedIndexChanged="SiteCombo_SelectedIndexChanged"
                            cssclass="dropDownNav"
                            AutoPostBack="true">
                        </asp:DropDownList>
                    </ContentTemplate>
                </asp:UpdatePanel>

Ok when I change RegionCombo it causes a full page reload?

Any ideas what I'm doing wrong?