Cant set onload of XMLHTTPRequest

Hi

I'm writing a extension for sunbird. For some reason I can't access
XMLHTTPRequest (), but I found this way to instanate a XMLHTTPRequest,
which works for syncroness calls.

var Request =
Components.classes["@mozilla.org/xmlextras/xmlhttprequest;1"].createInstance(Components.interfaces.nsIXMLHttpRequest);

But when using this method, when I set the onload event, I get this
error:
"Cannot modify properties of a WrappedNative".

Any ideas?


Gary