DeferredResult dont return value on ajax request

I´m using DeferredResult to make async request, and I´m using ajax on client side. But every time that I want to return one of this async request from another thread, the response that I receive in my client is not the one that I pass in the setResult

 deferredResult.setResult("foo");

And I only receiver status 200 ok. And looking in the firebug I dont see any response tab. Just an xml tab that says:

XML Parsing Error: no element found Location: moz-nullprincipa 

Am I doing something wrong?

Regards.