@each with multi-assign causes fatal error

[ghost]

using @each with multi assign as per Sass Reference. Using the scss from that page:

@each $header, $size in (h1: 2em, h2: 1.5em, h3: 1.2em) {
  #{$header} {
    font-size: $size;
  }
}

causes the following error:

com.vaadin.sass.internal.parser.SCSSParseException: Error when parsing file /path/file.scss
Encountered "," at line 1, column 14.
Was expecting one of:
<S> ...
"in" ...

The relevant bit of the stack trace appears to be

Caused by: com.vaadin.sass.internal.parser.SCSSParseException: Error when parsing file /private/var/folders/2y/pqy2wgh10zjcz2fq_x98fs0m0000gn/T/[PATH redacted]/zzz.scss
Encountered "," at line 1, column 14.
Was expecting one of:
    <S> ...
    "in" ...

    at com.vaadin.sass.internal.ScssStylesheet.get(ScssStylesheet.java:176)
    at com.vaadin.sass.SassCompiler.main(SassCompiler.java:92)
    at com.github.bordertech.wcomponents.sass.SassCompiler.callImpl(SassCompiler.java:58)
    at com.github.bordertech.wcomponents.sass.SassCompiler.main(SassCompiler.java:44)
    at com.github.bordertech.wcomponents.sass.SassTask.execute(SassTask.java:112)
    ... 132 more