The null value does not represent a number. A Number object does represent a number.

I reworked the error messages and now they provide given input, expected output and actual output.

Thank you for your feedback.

I'll look over the testcases and improve them. I don't know if I'll find time for it today, but as soon as they are ready I will let you know it.

There's no one right answer to this one. I do think that that particular solution is less clever than, say, mine, but there's nothing incorrect about it...