'toLowerCase': object is null or undefined in DomEvent() function

I have developed a web app in Visual Studio 2008, and when I run I consistently get the following error:

Microsoft JScript runtime error: Unable to get value of the property 'toLowerCase': object is null or undefined

and in the debugger it shows me the source file is "ScriptResource.adx?****[dynamic][Read Only]". The error is occurring in the DomEvent() function:

$type = Sys.UI.DomEvent = function DomEvent(eventObject) {
    var e = Function._validateParams(arguments, [
        {name: "eventObject"}
    ]);
    if (e) throw e;
    var ev = eventObject;
    var etype = this.type = ev.type.toLowerCase();                                         <== ERROR
    this.rawevent = ev;
    altkey = ev.altKey;

and DomEvent() is called from DomEvent$addHandler() in the same file:

$addHandler = $type.addHandler = function DomEvent$addHandler(elements, eventName, handler, autoRemove) {
    var e = Function._validateParams(arguments, [
        {name: "elements"},
        {name: "eventName", type: String},
        {name: "handler", type: Function},
        {name: "autoRemove", type: Boolean, mayBeNull: true, optional: true}
    ]);
    if (e) throw e;
    if (eventName === "error") throw Error.invalidOperation(Sys.Res.addHandlerCantBeUsedForError);
    Sys.query(elements).each(function() {
        var nodeType = this.nodeType;
        if (nodeType === 3 || nodeType === 2 || nodeType === 8) return;
        Sys.UI.DomEvent._ensureDomNode(this);
        if (!this._events) {
            this._events = {};
        }
        var eventCache = this._events[eventName];
        if (!eventCache) {
            this._events[eventName] = eventCache = [];
        }
        var element = this, 
            browserHandler;
        if (this.addEventListener) {
            browserHandler = function(e) {
                return handler.call(element, new Sys.UI.DomEvent(e));
            }
            this.addEventListener(eventName, browserHandler, false);
        }
        else if (this.attachEvent) {
            browserHandler = function() {
                var ex, ev = {};
                try {ev = Sys.UI.DomElement._getWindow(element).event} catch(ex) {}
                return handler.call(element, new Sys.UI.DomEvent(ev));                        <== HERE
        }
        this.attachevent('on' + eventname, browserhandler);
    }

Has anyone seen this type of behavior?