Modelpoup is not shown in IE7 but works fine in chrome

Question

 <asp:Panel ID="pnlAddBuilding" runat="server">

            <asp:UpdatePanel ID="upnlAddBuilding" runat="server">

                <ContentTemplate>

Controls here

   </ContentTemplate>

            </asp:UpdatePanel>

        </asp:Panel>

<ajaxToolkit:ModalPopupExtender ID="ModalPopupExtender1" runat="server" BackgroundCssClass="modalBackground"

    TargetControlID="btnModalPopup" PopupControlID="pnlAddBuilding" Enabled="True">

</ajaxToolkit:ModalPopupExtender>

<asp:Button ID="btnModalPopup" runat="server" Style="display: none;" />

i am trying to show in linkbutton click event like as follows
ModalPopupExtender1.show();

but it is not shown in IE7

it gives error UnknownRunTime Error 

plz any one help me