When I try to get info for a set of pages specified by a generator and that generator returns nothing, I'd expect to get an empty set. But you just get no set!
<api> <query> <allpages /> </query> </api>
<api> <query> <pages /> </query> </api>
Or is it a feature? I think its hard to detect when parsing the result, because you usually step through these elements when trying to get your pageset. Then you never know whether its an error in your request, an server error (api error) or its just empty results.