INVALID_STATE_ERR: DOM Exception 11

I'm developing a simple auxiliary class to send requests using XmlHttpRequest (code below). But I cant make it work. At google chrome, for example, I get the error INVALID_STATE_ERR: DOM Exception 11 and at the other browsers I get a status == 0.

//@method XRequest: Object constructor. As this implements a singleton, the object can't be created calling the constructor, GetInstance should be called instead
function XRequest() {
    this.XHR = XRequest.CreateXHR();
}
XRequest.instance = null;

//@method static GetInstance: Creates a singleton object of type XRequest. Should be called whenever an object of that type is required.
//@return: an instance of a XRequest object
XRequest.GetInstance = function() {
    if(XRequest.instance == null) {
        XRequest.instance = new XRequest();
    }
    return XRequest.instance;
}

//@method static CreateXHR: Implments a basic factory method for creating a XMLHttpRequest object
//@return: XMLHttp object or null
XRequest.CreateXHR = function() {
    var xhr = null;
    var factory = [
        function() { return new XMLHttpRequest(); },
        function() { return new ActiveXObject("Msxml2.XMLHTTP"); },
        function() { return new ActiveXObject("Microsoft.XMLHTTP"); }
    ];

    for(var i = 0; i < factory.length; ++i) {
        var f = factory[i];
        xhr = f();
        if(xhr) return xhr;
    }
    return null;
}

XRequest.prototype.SetRequestHeader = function(name, value) {
    if(this.XHR) {
        this.XHR.setRequestHeader(name, value);
    }
}

XRequest.prototype.SendRequest = function(args) {
    var async = true;
    var type = "";
    var url = "";
    var username = "";
    var password = "";
    var body = null;
    var success = null; 
    var failure = null;

    for(e in args) {
        switch(e) {
            case "async":
                async = args[e];
                break;
            case "type":
                type = args[e];
                break;
            c