Consequences of Exhaustion

I play poorly when I am tired. Nonetheless, I do a fair amount of tactics training late at night as I'm drifting off to sleep, and in the early morning before the first cup of coffee. Such training may be less productive than training while awake and focused. This evening I looked at this problem from the Anthology of Chess Combinations. Within a few seconds, I saw Black's idea for drawing the game. However, I overlooked a key resource for White. I made a hasty move and the Chess Informant Solver's Kit software told me the answer was wrong. Another minute's observation revealed White's resource, and then the correct first move of the problem became obvious.

Black to move

The problem is number 256 in the Anthology, and comes from Unzicker -- Averbakh 1952.