That is the downside, FZROX is only available to Fidelity customers. There’s also a potential risk it won’t track total market performance as well as predicted because one of the ways they keep the expense ratio at zero is by only purchasing what is supposed to be an equivalent subset of the total market to keep their transaction costs down. I don’t remember the specifics but I think they leave out lots of tiny companies and its not a significant risk because they couldn’t actually move the needle one way or the other.