Fastmail multipart/alternative not detecting

[tyrosinase]

Not sure if the example under walk_parts is at issue or if there's something funky about Fastmail, but using the example code doesn't return a true value for subparts. It will turn around and correctly say can't get body as a string for multipart/alternative if you ask it for the body_str of the supposedly-not-alternative email, so it's picking up the content type just fine.

There's probably something obvious I'm missing (it's been years since I played around with email) but I'm stumped. (There's also really nothing in the docs about how to deal with multipart/alternative outside of that example, which just returns.)