Sass function to add prefix before css class

Question

I'm trying to make a sass function that prefixes my class name, for example:

pf(.test) {
  display: block;
}

desired output:

.prefix-test {
  display: block;
}

What I've tried:

@function pf($class) {
    @return .#{$brand-prefix}-$class;
}

But I get an error

error reading values after @return Line 14 Column 9.

Any suggestions?