Neither sample tests nor the submission tests expect an Option. This was discussed in the translation discourse and it was decided that the inclusion of a few negative numbers just for the sake of including a few negative numbers was silly.
It is possible that you opened the kata in the small window when the translation was published, but the sample tests still had the Option as an expected result (see issue raised by Unnamed below). If so, you can either ignore the sample tests or reload/reset the kata to see the updated sample tests. I would suggest simply ignoring them. The expected return value is a String, so just use that.

Is your solution returning an Option?