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

the problem is some how $Util.IsChrome set's as true even though I am running IE 8.  Please let me know if anybody had same issue and fixed this issue.

 if($util.IsChrome || $util.IsSafari)
            {
                var onResizeFn = Function.createDelegate(this, this.__resizeTopLevelItems);
                $addHandler(window, 'load', onResizeFn);
                
                 //I.I. 90688: window.load is not called when part of the page is posted back by UpdatePanel
                 var prm = Sys.WebForms.PageRequestManager.getInstance(); --> it fails right there and doesn't load the webdatamenu.
                 if (prm) prm.add_endRequest(onResizeFn);
            }
            else
                this.__resizeTopLevelItems();
        }

it is under $IG.WebDataMenu.prototype =

thanks

ezpal