Unit lengths appears as illegal in postcss grammar

[felixsanz]

If you define a mixin like this:

@define-mixin square $dimension {
    width: $dimension;
    height: $dimension;
}

And you want to make use of it like:

.button {
  @mixin square 200px;
}

The px word appears as illegal character.

(Sorry for the sugass thing, ignore it)