'Sys is not defined' error when making web request during unload

I have an unload handler that makes a web request, and doesn't have a completed handler (it's a request that I want fired off before the next page loads, and I therefore don't care about the response). 

The request works fine but an Error will show up in FireFox as it loads the next page:

'Sys is not defined

   _this._webRequest.completed(Sys.EventArgs.Empty);  MicrosoftAjax.debug.js (line 4009)'

 Is this a bug or is there a workaround? 

Is there another way to specify a web request in which a response is not expected, which makes the executor handling the onreadystatechange event unnecessary?

 Thanks for any insight!

Mike