Null Reference Exception in authenticated Content Page

I have created a masterpage where I require user login. The content pages are only displayed if the user is authenticated.
<LoggedInTemplate>
                    <asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server">
                    </asp:ContentPlaceHolder>
</LoggedInTemplate>   
On my content page I have
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
<asp:DropDownList ID="dropdown1" runat="server"></asp:DropDownList>
</asp:Content>
When run, I can successfully authenticate and then when I attempt to view this particular content page I get an error on the page load where ...