They didn't state that when the question was asked though. It may be that this was made clear to them before the show, but to the viewers it was simply disallowed and an excuse made after the event. Darned poor presentation.
Well we are being told two different things here - one says they were told no proper nouns and another says they were only told afterwards - which is the correct version?
I have just had a look on the BBC iplayer and luckily it was the first question they had to answer. They were telt proper nouns were excluded BEFORE any answers had been given.