OSKI: Can't play word

In this game:

http://www.littlegolem.net/jsp/game/game.jsp?gid=1225261&nmove=14

I am unable to play the word "Powders" at move 15. Does anybody have an idea why? I find the word in the "Sowpods" list, and as far as I can see, it does not contain any word already played?

That's because the initial word was "pow" and "powdres" includes "pow".

Ah, yes - the initial word, of course! Thanks! I always forget about that, since it's not listed among the other played words. I think it should be included as a forced move 1, like in Gomoku.