Can't choose a block in the fill command

There's a bug in the fill command.

This bug causes players to not be able to pick a block other than the original block that goes with it.

For instance, if I did;

"/fill ~ ~-1 ~ ~ ~-1 ~ concrete"

It would fill the block directly under me with white concrete.

BUT

If I wanted to do a different colour like;

"/fill ~ ~-1 ~ ~ ~-1 ~ concrete 5"

No block would be placed, and you get a syntax error saying "Syntax error: Unexpected "5" at "Concrete >>5<<"'

This is very annoying and is an issueif we try to use the /fill replace command.

I do apologise if someone covered this already, I couldn't find any, so I made it myself.