error with gulp-autoprefixer 2.0.0 #26

gulp.task('sass', function(){
   return gulp.src('app/assets/styles/b.scss')
    .pipe($.plumber())
    .pipe($.rubySass({
      style: 'expanded',
      precision: 10
    }))
    .pipe($.autoprefixer({browsers: ['last 1 version']}))
    .pipe(gulp.dest('.tmp/styles'));
});

Plumber found unhandled error:
 CssSyntaxError in plugin 'gulp-autoprefixer'
Message:
    /Users/oych/evolution/b.css.map:3:1: Unknown word
Details:
    reason: Unknown word
    file: /Users/oych/evolution/b.css.map
    line: 3
    column: 1
    source: {
"version": 3,
"mappings": "",
"sources": [],
"names": [],
"file": "b.css"
}
    fileName: /var/folders/cv/tzl5b7s559j104k1llztqrcr0000gn/T/gulp-ruby-sass/_14139e58-9ebe-4c0f-beca-73a65bb01ce9/b.css.map