Piston Extension cannot be '/testforblock'ed or replaced by /fill

Piston Extension cannot be '/testforblock'ed or replaced by /fill

When you do '/setblock ~ ~ ~ piston_extension' and then '/testforblocks ~ ~ ~ air' it says it found 'tile.null.name'. When you press F3 and look at the block if you look to the right of the screen it says that there is a piston_extension block so this is a bug!