LG won't let me move

I am playing black in this game and would like to move at a1 (lower left point), but the site will not let me move there even though it is a legal move.

http://www.littlegolem.net/jsp/game/game.jsp?gid=1664141

It seems strange to tell a much stronger player about the rules, but to me this looks like a normal ko situation, hence an immediate a1 is NOT a legal move. Or am I missing something?

Rex, the linked game shows you as being White (The Tible as Black)...?

At move 50, after 49 b b1, a1 was illegal by ko. (Don't know when exactly you posted this.) Now it is legal again.