Faerie dragon strangled

Assassin in the elven vaults managed to strangle faerie dragon despite his sleep immunity. In this case, it was a moot point since he attacked anyway, but I wonder if it indicates a bug that would make him able to be knocked out in other circumstances as well. You tell your familiar to 'west'. A faerie dragon has arrived. Someone grabs hold of a faerie dragon's neck and puts it to sleep. A faerie dragon is surrounded by a pink outline. The jade elf assassin steps out of the shadows.