[Zope] Problem with alternate path and request

Hello list, I'm using VHM and I need the (real) remote IP, as given by HTTP_X_FORWARDED_FOR. However, when accessing the page without any virtualization, I need to fall back on REMOTE_ADDR. Now, any idea why the following <p tal:content="request/HTTP_X_FORWARDED_FOR | request/REMOTE_ADDR"></p> doesn't work as expected: HTTP_X_FORWARDED_FOR returns an empty string rather than failling in favor of REMOTE_ADDR. I've overcomed the problem using "or" in python, but I got puzzled with this situation. Is this a bug or am I missing something? Cheers, Fernando