Certain CSS level 4 selectors are incorrectly compressed

[nex3]

From sass/sass#2497:

I have a CSS level 4 selector in SCSS:

.someclass:nth-child(1 of .due-today) { 
/* ... */
}

When compressing the CSS via Sass, the result is something like…

.someclass:nth-child(1of.due - today) {
/* ... */
}

…which obviously isn't working.