Allegroserve doesn't seems to provide USER AGENT information

There is a slot in the REQ data structure (NET.ASERVE:HTTP-REQUEST)

called REQUEST-HEADER-USER-AGENT

but whenever I examine that slot it is NIL.

As I understand it a USER AGENT would be an id for the browser that
sent the http request (e.g., Mozilla, IE)

Unfortunately, this seems like a useful piece of information when composing
certain web pages.

So why is it NIL, and is there some other way to get at this information?

Thanks.