yes ;)

Random Tests, System.NullReferenceException : Object reference not set to an instance of an object.

There is your answer. Sample tests are just that - examples. Many more tests when you hit 'attempt', usually random. It's hard to tell where the error comes from, but it's from your code. Start by reading THIS.

that's because the full test suite (using "attempt") contains more tests then the sample tests. You'll find some bit of information there.

=> first things first: print the inputs / try to get why your code fails on that input (edit: reading the error message is actually quite useful in this kind of situation... ;o )