Patronus question...shouldn't be spoilerish

Ok, so I am rewatching the Harry Potter movies, though I realized tonight I don't have Chamber of Secrets on dvd for some reason. Anyhow, so skipping Chamber of Secrets I am continuing on to Prisoner of Azkaban. I just rewatched the scene where Lupin is showing Harry how to make a partonus charm, using the boggart to replace the real dementor...

So then, if the incantation for a boggart is "riddikulus" and thinking something funny, how come the patronus charm worked on the boggart dementor instead of the "riddikulus" incantation?