$ plex-axi playlist add 501 --key 900 --write
environment: writable
exit: 1

error: "cannot add items to 'Example Playlist': those items are not all the same kind of media"
code: MIXED_MEDIA_TYPES
help[3]:
  Plex holds one media type per playlist, and an audio playlist takes music only
  Run `plex-axi track <key>` on each of 900 to see which is not music
  A rating key from a film or photo library resolves here but cannot go in
