Something I've wondered: how is this reconciled with the notion of "ignorance of the law isn't a defense"? If you do something that's a crime but have no idea t

Something I've wondered: how is this reconciled with the notion of "ignorance of the law isn't a defense"? If you do something that's a crime but have no idea that it's a crime (and had no intention to commit one) then you can generally still get convicted because that's not a defense, right? So where does the guilty mind come into play?