Harry Potter: 15 WORST Plot-Holes In Wizarding World Movies

14. How Does Expecto Patronum Work On A Boggart?

As established the first time we meet one, the only way to defeat a Boggart is the spell Riddikulus and an image from its target of something ridiculous to stop its powers of fear. It is then defeated by the laughter inspired, since it seemingly feeds on emotion.

With that established, how come in The Prisoner Of Azkaban, Professor Lupin uses a Boggart to train Harry to use the Patronus charm? At that point, Harry is crippled by his fear of the Dementors, so it makes sense that he'd benefit from some immersion therapy, but there's a problem: using a Patronus simple wouldn't work.

The Patronus charm would have no effect on a Boggart, since it's entirely the wrong spell and yet the boggart in that form is defeated when Harry successfully casts one. So what gives?