Can use /fill ~ ~ ~ ~ ~ ~ minecraft:string in command blocks

can't use the fill command with minecraft:string

I was expecting the floor to turn to string (we were making a spawn teleporter)

however the command block and my chat command said that minecraft:string does not exist

Steps to Reproduce:
1. Place command block/open chat
2. type in /fill X Y Z X Y Z minecraft:string
3. activate command block/press enter

NOTE tried on both my mac and P.C. on both a realm and single player world