Modal Popup extender giving javascript error object required

[ja3_bhende]

I am usind ajax modal popup extender control.I used following code.

</cc1:ModalPopupExtender>

<asp:Panel ID="pnlConfirm" runat="server" Width="328px" Visible="False" CssClass="modalBackground" >

<asp:Label ID="lblConfirm" runat="server" Text="No employee is marked as absent,do you want to continue?" Width="409px"></asp:Label><br />
<br />
<asp:Button ID="btnYes" runat="server" Text="Yes" CausesValidation="false" UseSubmitBehavior="false" CssClass="button" Width="34px" />

<asp:Button ID="btnNo" runat="server" Text="No" CssClass="button" Width="27px"/></asp:Panel>

<asp:Button runat="server" ID="btnShowModalPopup" style="display:block"/>
<cc1:ModalPopupExtender ID="modalConfirmPopUp" runat="server"
TargetControlID="btnShowModalPopup"
PopupControlID="pnlConfirm"
DropShadow="true"
CancelControlID="btnNo"
OkControlID="btnYes"
>
</cc1:ModalPopupExtender>

but its giving me javacript error as follows-
Webpage Script Errors

User Agent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; .NET CLR 2.0.50727; InfoPath.1)
Timestamp: Thu, 25 Mar 2010 11:30:28 UTC

Message: Object required
Line: 72
Char: 9
Code: 0
URI: http://localhost/NewProject/ScriptResource.axd?d=9p6g4WEDkF16Jipsu7vz2Id8pFW8DJwssHl8hJjPmiNRX_pKrajHf7X9Nl3H_ctQbP-Fn4xONOFOEdRgPCOoafjIsmeRTZYabf9ZNqhvEOg1&t=633050947360000000

The same code worked fine in another form.Please solve this problem.