suddenly change in /fill or bug?

I was developing my world/server with /fill I put the correct coordinates and the block would be ''dark_oak_stairs'', however I noticed that when I put the position of the block that would be from 0 to 5 the game simply says that this command is wrong, and this sudden change discouraged me from continuing my world... get it back to normal...  (how do i choose the position of the blocks now?