This has been fixed

Thank you very much, now that I can try and fix. I'll be amending my post to spoiler in the meantime.

Actually, the assertions have the parameters in the wrong order, so you need to read "Expected x, but was y" the other way around.

I can only imagine the reason being the way Java is converting the hyphen char to an int. If that's assuming a value of -1 for the -, it would be adding a 1 to the beginning of your returned string.