This error does not try to tell you that tests think you are cheating. On the contrary, there are some tests titled "No cheating!" (maybe not the most fortunate title for a test group), and all of them pass. It means that tests did not detect any disallowed things in your solution.

The error you get is indeed not really clear. I was able to reproduce it returning None from my solution, so I expect you have some branch in your code which falls off the function without returning anything.

That's not cheating. The tests in that kata check that you're not using eval and similar work-arounds.