X' was working at a place over which a crane swung carrying heavy stones. Both 'X' and his employer knew that the crane so swung and recognized the possibility of danger if a stone fell from the crane. No warning was given 'X' of the swinging time of the crane whenever this took place. Once a heavy stone fell from the swinging crane and injured 'X'. In a claim for torts, the employer pleaded that there was no liability on the basis of the principle volenti non fit injuria. This plea-