I don't care that mine or other users solutions will become obsolete. Fix this.

Please update this it stopped me for way to long!

I know O:) But many users had already solved the kata and now changing that won't makes sense, as all the previous solution will become obsolete :(

Forcing it to be a string is a terrible idea.
Javascript provides you with a NaN value specifically for this sort of case.

OOx is right you need to return NaN as a string, I have also specified that in the description.

this confused me for a bit - it should be 'NaN' as a string, not actually NaN, being returned

Please specify which test cases are not working, better if you copy all the content of the output window and paste it here when the error occurs.