Can't unlock mystery champion, despite having 10 to choose from? (says i only have 9??)

Hey guys, anyone know why I'm getting that error about not having enough items to choose from for a mystery item, if I have 10 champs left to unlock? Are certain ones not eligible for mystery champ?? (I do have Zoe, so it's not just a "newest" champ thing). Is Kayne excluded from the list for mystery unlocks too?