AsyncPostBackTrigger is not a known element?

I'm using asp.net and c#, making a page with ajax updatepanels. When I try and insert the trigger element, I get the error message AsyncPostBackTrigger is not a known element. What am I missing?

<asp:UpdatePanel ID="UdpEPL" runat="server" UpdateMode="Conditional" 
    Visible="False">
    <开发者_开发问答ContentTemplate>
        <Triggers>
            <asp:AsyncPostBackTrigger ControlID="BtnEplShowSubmit"
                EventName="BtnEplShowSubmit_Click"/>
        </Triggers>
    </ContentTemplate>
</asp:UpdatePanel>

---

Remove the Triggers section from your ContentTemplate:

<asp:UpdatePanel ...>
    <Triggers>
       <asp:AsyncPostBackTrigger .../>
    </Triggers>
    <ContentTemplate>

    </ContentTemplate>
</asp:UpdatePanel>