I was playing a game that got to that ending and I found that you can't claim a draw. Your opponent must accept the draw. Is this a bug?

No, it is still possible to mate.

Either player could just stop moving. http://blog.chess.com/webmaster/live-chess-gets-some-love

"Automatic draw on time-out if opponent has insufficient mating material: KB, KN or KNN"

Be careful if the Fair Play policy considers this stalling.

There's almost no chance that's going to happen in a game. You could even throw away your knight to prevent that.

That's a better way. Much faster.

http://www.fide.com/component/handbook/?id=124&view=article

9.6  The game is drawn when a position is reached from which a checkmate cannot occur by any possible series of legal moves. This immediately ends the game, provided that the move producing this position was legal.

49...Ke8 was worth a shot by black in that game.

(But how do you throw away your knight ? He doesn't have to take it if you offer it, unless it's a fork check)

Well you've just answered your own question :)

I was actually posing the question for Anthony (thought that would have been obvious)