Exactly, you're supposed to return False for that one.

@Beaxx check my solution here. It's semantically identical to the second ranked best practice submitted by zebulan here.

In zebulan's comments there is discussion regarding how it works and why it's better practice.

So what's better then?

I beg to differ, as bad practice is to modify input in place if not required, not to just pass objects which are mutable.

That could be a suggestion, at best, but I would not call it an issue, otherwise you should flag A LOT of other katas.

And, mh... it is quite debatable: what would you pass in a similar problem?