Modal Popup Extender doesn't work

I'm using Ajax ModalPopupExtender, but when I run the application the popup doesn't show. It gives following error in Chrome error console:

Uncaught TypeError: Cannot read property 'UI' of undefined

My .aspx page code is:

<div><asp:ScriptManager ID="scm" runat="server"></asp:ScriptManager><asp:Button ID="btnShow" runat="server" Text="Show Modal Popup" /><asp:ModalPopupExtender ID="mp1" runat="server" PopupControlID="Panel1" TargetControlID="btnShow" OkControlID="btnClose"></asp:ModalPopupExtender></div><div><asp:Panel ID="Panel1" runat="server" CssClass="modalPopup" align="center" style = "display:none">
    This is an ASP.Net AJAX ModalPopupExtender Example<br /><asp:Button ID="btnClose" runat="server" Text="Close" /></asp:Panel></div>