asp.net - asp.net ModalPopupExtender not behaving properly

Question

I have the following code on a .aspx file. If I comment out the ModalPopupExtender code works fine.:

   <cc1:ModalPopupExtender BehaviorID="mdlPopup" runat="server" TargetControlID="btn"
        ID="mdl1" PopupControlID="pnlPopup" OkControlID="LinkButton1" BackgroundCssClass="modalBackground" />
    <asp:Panel ID="pnlPopup" runat="server" CssClass="confirm-dialog" Style="display: none;">
        <div class="inner">

          Work Order #: <asp:TextBox ID="txtWorkOrder" runat="server"  Width="285" Text=""></asp:TextBox><br /><br />

            Please Enter Corrective Actions<br />
            <asp:TextBox ID="TextBox2" runat="server" Height="150" Width="285" TextMode="MultiLine"
                Text=""></asp:TextBox>
            <asp:Button ID="btnOK" runat="server" Font-Bold="true" Text="Submit" OnClick="click"
                Width="150" />
            <asp:LinkButton ID="LinkButton1" runat="server" CssClass="close" OnClick="cancel" />
        </div>
    </asp:Panel>
    <asp:Button ID="btn" runat="server" Style="display: none;" />
    <asp:Label ID="id" runat="server" Text="Label" Visible="false"></asp:Label>

The problem that I am having is that the page just hangs when I have this code in the .aspx file. Is there anything I should do. It does not even get to the Page_Load.

I am calling this page that has the above code with the following:

    string url = "Alerts.aspx?";
    url += "ID=" + id.Text;
    Response.Redirect(url);