Gulp任务无效

Question

我想缩小并重命名文件夹中的所有.css（../dist/css），但它说有错误（您可以在下面的屏幕截图中看到错误）。

var gulp =         require('gulp');
var postcss =      require('gulp-postcss');
var autoprefixer = require('autoprefixer');
var lost =         require('lost');
var rucksack =     require('rucksack-css');

var cssnano =      require('cssnano');
var rename =       require('gulp-rename');

gulp.task('css', function() {
    var plugins = [
            autoprefixer({
                browsers: ['last 2 version']
            }),
            rucksack({
                quantityQueries: false,
                alias: false,
                easings: false,
                inputPseudo: false
            }),
            lost
    ];
    return gulp.src('app/styles/**/*.css')
        .pipe(postcss(plugins))
        .pipe(gulp.dest('dist/css'));
});

  

// ========这部分不起作用============

 gulp.task('css-min', ['css'], function() {
  return gulp.src('app/styles/*.css')
      .pipe(cssnano())
      .pipe(rename({suffix: '.min'}))
      .pipe(gulp.dest('dist/css')); });

gulp.task('watch', ['css', 'css-min'], function() {
        gulp.watch('app/styles/**/*.css', ['css']);
});

 gulp.task('default', ['watch']);