Nope, that's not it; thanks for the suggestion though.

I ought to point out that these props I'm doing work fine under P4 and DAZ|Studio, just as they always have done. It's just that I want to use the more advanced materials in P6 and yet when I apply them I just can't render them until I put some other object into the scene.

I don't even know if the props I'm making work in other people's versions of P6, maybe they do. I made a pair of glasses for V4 which I gave away here: http://myweb.tiscali.co.uk/planetvoodoo/RetroSpecsFree.zip 
and they appeared to work to me at first because I just left the frames untextured black anyway, however later when I tried altering them I came across this problem. They work with V4 okay, but they don't seem happy on their own, which is a worry. No-one's given me any feedback on the freebie, so I sort of assumed they must work for other folks but I just don't know. If not, I'd like to know about it and what I have to do to fix it because I'm planning on selling a pack of higher-quality glasses models soon and obviously, they need to work correctly.