UpdatePanel does not have a public property named 'TextBox'.

I'm looking to update the contents of a panel using an asp:updatepanel. I get an error on the page saying: DotNetNuke.Services.Exceptions.ModuleLoadException: Type 'System.Web.UI.UpdatePanel' does not have a public property named 'TextBox'.

See code below:

    <asp:ScriptManager runat="server" ID="ScriptManager" />
    <asp:UpdatePanel runat="server" ID="brandAddingContainer" Visible="false">
        <ContentTemplate>
            <asp:LinkButton runat="server" ID="brandAddingPrompt">
                <img src="/images/add.gif" alt="Add New Brand" onclick="addNewBrand_clicked"/> Add New Brand
            </asp:LinkButton>

            <asp:Panel ID="sendNewBrand" runat="server">
                <asp:TextBox runat="server" ID="newBrandName"></asp:TextBox>
                <asp:Button runat="server" ID="sendBrandName" Text="Add Brand" OnClick="sendNewBrand_clicked" />
                <asp:Label runat="server" ID="insertionFeedback" Visible="false" />
            </asp:Panel>
        </ContentTemplate>

        <Triggers>
            <asp:AsyncPostBackTrigger ControlID="brandAddingPrompt" EventName="Click" />
        </Triggers>
    </asp:UpdatePanel>