figured this out (I have done other things since then..) and was that I'd used gsub inplace, so lesson learnt.
Maybe.

cheers anyway :)

Testing for "pySswfJRC ddF VMMkYL"
It should work for random inputs too -  Expected: "You just wanted my autograph didn't you?", instead got: "pySswfJRCddFVMMkYL"

unless I'm missing something (it is late here...), the random Ruby tests arent working properly - they seem to expect "You just wanted my autograph didn't you?" for all of them.
But then if I try and just pass through the random ones, it fails for the correct reason

Testing for "FeVe OwIQ UmXa"
It should work for random inputs too -  Expected: "FeVeOwIQUmXa", instead got: "FeVe OwIQ UmXa"

Theres going to be something really obvious I've done wrong...

Thats exactly what I was doing too :)

I had to check for various non-numbers types so maybe worth printing out the value and type at the top to see what is failing in particular, if you haven't

cor, that was quite hard work! With my solution as well the best optimisation I found appeared to be using python3 - saved about a third of the time... Anyways, cheers for the challenge :)

cool :)

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

Hi, is it just me or are the 2 assumptions in the description actually needed?

You didn't call lower()