If you're talking about the attempt it's working fine. I just finished it a few minutes ago. The other two test cases are to make sure your solution actually works. I'm hinting towards the hint here as they also test if you handled what the hint specified. I was stuck on that and thought just like you it was broken till I really thought about the hint again. If you look at the stack trace and see something other than NPE then this doesn't apply, else it does.