Sourcemaps not working with autoprefixer

Gulpfile.js

var sass         = require('gulp-ruby-sass');
var plumber      = require('gulp-plumber');
var sourcemaps   = require('gulp-sourcemaps');
var postcss      = require('gulp-postcss');

gulp.task('sass', function() {
    return gulp.src('uncompressed/scss/*.scss')
    .pipe(plumber({
      errorHandler: onError
    }))
    .pipe(sass({sourcemap: true, sourcemapPath: '../../uncompressed/scss', style: 'compressed'}))
    .pipe(sourcemaps.init())
    .pipe(postcss([autoprefixer]))
    .pipe(sourcemaps.write('.'))
    .pipe(size({title: 'css'}))
    .pipe(gulp.dest('assets/css'));
});