Just my two cents but I would go against the first interaction with the user being find the bug, I didn't didn't find it and missed the point, granted I didn't take much time and I'm on mobile, but I guess that's the reality for many people if not most :)
The typo is in the name bill in the value of publicKey. I assume this is a way to introduce the user to definitions that help you avoid these kind of mistakes. I think it would be more suitable as a use case description rather than an exercise for the user.
