A control with id could not be found for trigger in update panel

Hello ,

I am posting the accordion pane in update panel , and in that pane there are some of the buttons but, i am getting the error when i use the triggering for the button click event.

Here is Source Code:

<asp:UpdatePanel ID="UpdatePanelacc" runat="server" UpdateMode="Conditional">
<Triggers>
<asp:AsyncPostBackTrigger ControlID="btnCal" />
</Triggers>
<ContentTemplate>
<table width="100%">
<tr>
<td>
<cc1:Accordion ID="Accordion1" runat="server" HeaderCssClass="headeraccordian">
<Panes>
<cc1:AccordionPane ID="AccordionPane1" runat="server">
<Header>
<asp:Label ID="dailyreport" runat="server" Text="Daily Report"></asp:Label>
</Header>
<Content>
<div>
<asp:TextBox ID="txtDate" runat="server" ReadOnly="True" Width="94px"></asp:TextBox>
<asp:Button ID="btnCal" runat="server" Text="Select Date" OnClick="btnCal_Click" />
&nbsp;
<asp:Button ID="btnDownLoads" runat="server" Text="Download" OnClick="btnDownLoads_Click" />
<asp:Calendar ID="cal" runat="server" OnSelectionChanged="cal_SelectionChanged" BackColor="White"
BorderColor="White" BorderWidth="1px" Font-Names="Verdana" Font-Size="9pt" ForeColor="Black"
Height="190px" NextPrevFormat="FullMonth" Width="350px">
<SelectedDayStyle BackColor="#333399" ForeColor="White" />
<TodayDayStyle BackColor="#CCCCCC" />
<OtherMonthDayStyle ForeColor="#999999" />
<NextPrevStyle Font-Bold="True" Font-Size="8pt" ForeColor="#333333" VerticalAlign="Bottom" />
<DayHeaderStyle Font-Bold="True" Font-Size="8pt" />
<TitleStyle BackColor="White" BorderColor="Black" BorderWidth="4px" Font-Bold="True"
Font-Size="12pt" ForeColor="#333399" />
</asp:Calendar>
</div>
</Content>
</cc1:AccordionPane>

</Panes>
</cc1:Accordion>
</td>
</tr>
</table>
</ContentTemplate>
</asp:UpdatePanel>

Kindly provide me the solution for this.