/fill will not let you use string.

When I wanted to put a huge cube of sand on string, I used /fill to try to put string there, but when I typed the coordinates then put string and did enter, it did the 'unexpected' error.

Steps to reproduce

1. Do /fill ~~~ ~~~ string
2. Press enter

What I observed

It gave the 'unexpected' error

What I expected

It would fill string wherever you are

Note

T**his is only for /fill not for /setblock because you can do string on /setblock but not for fill