Command block teams problems

When attempting to join a team called "player" I get error "Could not add 1 player(s) to team player: EnchantedDiamond" or "An unknown error occurred while attempting to perform this command"
How To Replicate:
I tried to replicate it on a new world and the first time it happened again but the second time it worked, even tho I did the same thing although it still doesn't work on the first world