错误来自postcss插件，我想我可能写错了。
我正在尝试将cssnano和autoprefixer添加到postcss插件中。

gulp/node_modules/gulp-postcss/node_modules/postcss/lib/processor.js:143
        throw new Error(i + ' is not a PostCSS plugin');
        ^

Error: [object Object] is not a PostCSS plugin
    at Processor.normalize (/Applications/XAMPP/xamppfiles/htdocs/sites/gulp/node_modules/gulp-postcss/node_modules/postcss/lib/processor.js:143:15)
    at new Processor (/Applications/XAMPP/xamppfiles/htdocs/sites/gulp/node_modules/gulp-postcss/node_modules/postcss/lib/processor.js:51:25)
    at postcss (/Applications/XAMPP/xamppfiles/htdocs/sites/gulp/node_modules/gulp-postcss/node_modules/postcss/lib/postcss.js:73:10)
    at Transform.stream._transform (/Applications/XAMPP/xamppfiles/htdocs/sites/gulp/node_modules/gulp-postcss/index.js:47:5)
    at Transform._read (_stream_transform.js:167:10)
    at Transform._write (_stream_transform.js:155:12)
    at doWrite (_stream_writable.js:300:12)
    at writeOrBuffer (_stream_writable.js:286:5)
    at Transform.Writable.write (_stream_writable.js:214:11)
    at DestroyableTransform.ondata (/Applications/XAMPP/xamppfiles/htdocs/sites/gulp/node_modules/gulp-sass/node_modules/through2/node_modules/readable-stream/lib/_stream_readable.js:531:20)
Mac-a45e60e72dad:gulp JoeKonst$

我的代码：

// Dependancies
var gulp = require('gulp'),
    browserSync = require('browser-sync'),
    plumber = require('gulp-plumber'),
    autoprefixer = require('gulp-autoprefixer'),
    uglify = require('gulp-uglify'),
    compass = require('gulp-compass'),
    rename = require('gulp-rename'),
    nano = require('cssnano'),
    del = require('del'),
    postcss = require('gulp-postcss'),
    sass = require('gulp-sass');

// Styles
gulp.task('styles', function(){
    gulp.src('sass/main.scss')
    .pipe(sass())
    .pipe(postcss([autoprefixer({browsers: ['last 2 versions']}), nano()]))
    .pipe(gulp.dest('css/'));

    gulp.watch('sass/**/*.scss', ['styles']);
});

// Tasks
gulp.task('default', ['styles']);