ToME: the Tales of Maj'Eyal

On entering the dark crypt, I found that the left and top sides of the map had no border. This screenshot shows the fully explored map - directly to the left of my position is the edge of the level: http://i.imgur.com/aOxW1.png

Trying to move directly into the void does nothing, trying to move diagonally into it moves further along the corridor in that direction instead.

Edit: the same issue repeated itself on some of the later crypt floors, too.