Was this a teaching game (i.e.: a game where both parties agreed it was a lesson)? If it was and playing-out failed invasions was part of your teaching plan, you must have instructed your pupil in advance about this feature.
It is analogous to explaining to a pupil what handicaps are and why they are a good idea before starting a teaching game. I have done so several times. Some wanted an even game and that was fine by me.