I got this output (and failed the final tests) in exactly the same way because I was trying to use string() to cast a rune. In order to pass the randomized test, the code needs to be able to account for runes greater than a single byte.

I actually only had to make minimal changes to my code to pass the tests otherwise, so I'm leaving this for anyone else who may be tripped up by this - hopefully not spoiling too much :)