I changed theOutputFormatter to use string methods fromHelper to avoid splitting multi-byte UTF-8 characters and causing the "Invalid UTF-8" exception as a result. Unfortunately, it turns out thatHelper::length andHelper::substr are not mutually compatible for UTF-8, becauselength returns the length in graphemes, whilesubstr worked with characters. I decided to fix that, which is technically a BC break (hopefully not too severe). I considered making a private fixed copy of the method, but that just seemed weird.