floating in space… Ladies and Gentlemen, we are

ffmpeg aspect gotcha

Struggled with ffmpeg trying to re-encode an old video. Got this error:
Incorrect aspect ratio specification.
Then realized ffmpeg doesn't like floats when used with a colon as parameter for -aspect.
Using -aspect 2.42 or -aspect 242:100 is fine, but -aspect 2.42:1 is not. :/
Doh!