Syntax for Mismatch Tolerance

[RichardBray]

gulp.task('phantomcss', function (){
  gulp.src('./casper-tests.js')
    .pipe(phantomcss({
      mismatchTolerance: 0.5,
      screenshots: './test/styles/screenshots'
    }));
});

This doesn't seem to be working for me. Is the syntax incorrect, or does this not exist.