DropDownList is causing full page post back in UpdatePanel (Inside Repeater)

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ASP.NET

Ajax

I have a user control called AddressEdit, that contains two drop down list boxes inside a update panel like this:

<asp:UpdatePanel ID="upCountry" runat="server" >
    <ContentTemplate>
   <tr class="row">
            <td class="item-name">
                <%#GetLocaleResourceString("Account.Country")%>:
            </td>
            <td class="item-value">
                <asp:DropDownList ID="ddlCountry1" AutoPostBack="true" runat="server"  OnSelectedIndexChanged="ddlCountry1_SelectedIndexChanged"
                    Width="137px">
                </asp:DropDownList>
            </td>
        </tr>

        <tr class="row">
            <td class="item-name">
                <%#GetLocaleResourceString("Account.StateProvince")%>:
            </td>
            <td class="item-value">
                <asp:DropDownList ID="ddlStateProvince1" AutoPostBack="False" runat="server" Width="137px">
                </asp:DropDownList>
            </td>
        </tr>
   </ContentTemplate>
</asp:UpdatePanel>

And I am using this user control in a repeater.

<asp:Repeater ID="rptrBillingAddresses" runat="server">
            <ItemTemplate>
                <div class="address-item">
                    <CtrlAddressDispla:AddressDisplay ID="BillingAddresses" runat="server" Address='<%# Container.DataItem %>'
                        ShowEditButton="true" ShowDeleteButton="true" />
                </div>
            </ItemTemplate>
            <SeparatorTemplate>
                <div class="clear">
                </div>
            </SeparatorTemplate>
        </asp:Repeater>

Now problem is that, whenever I change country in ddlCountry its causing full page post back instead of partial postback.

How to resolve this problem?