Totally agree because of math.

I think this kata deserves a higher rank, since is pretty difficult for a 6 kyu

There's a test for Ruby that passes a nil value, but the description says nothing about this case. It only metions about an empty tree which is slightly different imo.
However, it's really nice kata, and will be even nicer if gets an improved description and some sample tests.

After I could see @Blind4Basics's answer I was able to find and fix the problem. IMO it worths mentionning in the description, that the source array shouldn't be mutated.

I'm hitting the same issue. All random tests seem to expect empty result set. Can you please share what solved the issue for you?