/fill not working on superflat

so i found a thing that the /fill command will not work for some reason on super flat worlds.. i do not know why, but each time I would, no matter how i typed it, weather I did the numbers, the ~~~, or a mix, it would always say "cannot place blocks out side of world", even though i would be above ground.... idk what's happening or maybe if its just my game specifically, but its making it rather difficult to do large builds