title: AsyncPostBackTrigger is not a known element?
body: <p>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 <em>AsyncPostBackTrigger is not a known element</em>. What am I missing? </p>
  
  <pre><code>&lt;asp:UpdatePanel ID="UdpEPL" runat="server" UpdateMode="Conditional" 
      Visible="False"&gt;
      &lt;ContentTemplate&gt;
          &lt;Triggers&gt;
              &lt;asp:AsyncPostBackTrigger ControlID="BtnEplShowSubmit"
                  EventName="BtnEplShowSubmit_Click"/&gt;
          &lt;/Triggers&gt;
      &lt;/ContentTemplate&gt;
  &lt;/asp:UpdatePanel&gt;
  </code></pre>