Keyword madness

[mercphoto]

This is driving me up the wall. Are there just certain words that Smugmug will not accept as a keyword? I have several race photos where I can't identify a rider, so I want to keyword them not with a number, but with "unknown". But that does not appear to be accepted. Then I tried "unknown_rider". Same deal.

Why in the world will this not work?????

CORRECTION: unknownrider did work. But unknown never did.