Sys.WebForms.PageRequestManager' Is Null Or Not An Object / Showing Error Message

if i implement updateprogress in my web page i got the following error messge

Microsoft JScript runtime error: 'Sys.WebForms.PageRequestManager' is null or not an object

.aspx

<link
href="cssUpdateProgress.css"
rel="stylesheet"
type="text/css"
/>
<script
type="text/javascript"
language="javascript">
var ModalProgress =
'<%= ModalProgress.ClientID %>';
</script>
<asp:ScriptManager
ID="ScriptManager1"
runat="server"
EnablePartialRendering="False"
LoadScriptsBeforeUI="False"
ScriptMode="Release"
>
<
</asp:ScriptManager>script
type="text/javascript"
src="jsUpdateProgress.js"></script>
<asp:Panel
ID="panelUpdateProgress"
Width="105px"
Height="25px"
runat="server"
CssClass="updateProgress">
<asp:UpdateProgress
ID="UpdateProg1"
DisplayAfter="0"
runat="server">
<ProgressTemplate>
<table
style="height:25px;">
<tr><td><img
height="21px"
width="15px"
src="imagesprocessing.gif"
style="vertical-align: middle"
alt="Processing"
/></td><td
valign="middle"
>Processing...</td></tr>
</table>
</ProgressTemplate>
</asp:UpdateProgress>
</asp:Panel>
<cc1:ModalPopupExtender
ID="ModalProgress"
runat="server"
TargetControlID="panelUpdateProgress"
BackgroundCssClass="modalBackground"
PopupControlID="panelUpdateProgress"
/>
javascript

Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(beginReq);
Sys.WebForms.PageRequestManager.getInstance().add_endRequest(endReq);

function

beginReq(sender, args) {// shows the Popup
$find(ModalProgress).show();
}

function

endReq(sender, args) {// shows the Popup
$find(ModalProgress).hide();
}