We are not fully sure about the C# version being correct. We revised it a couple of times before aproving it, and at first sight it seemed that the solution was correct. We don't know the translation author and we are not experts in the language, we would appreciate any help in order to improve the C# version.

We have removed the null input for validation purposes.
We are sorry for that mistake we are rookies in the kata-making field.
Thanks for the suggestion c:

Possible Java translation