/fill does not work with minecraft:string

I was trying to use this command:

/fill 1011 144 597 1017 144 591 minecraft:string

It detects an error at "minecraft:string

What I expected to happen was...:
That the area would be filled with string.

What actually happened was...:
I got a error message saying there was a error with "minecraft:string".

Steps to Reproduce:
1. use the /fill command
2. select an area
3. make sure the block is minecraft:string