新的咕哝警告:任务;concat，uglify“;找不到

正如标题所说，我是Grunt的新手。我正在学习位于以下位置的教程：http://24ways.org/2013/grunt-is-not-weird-and-hard/.这是一个较老的教程，但大多数似乎都是一样的。我安装了"grunt contrib concat"answers"grunt contrib uglify"，可以分别运行。但当我运行grunt时，我会得到以下错误：Warning: Task "concat, uglify" not found. Use --force to continue. Aborted due to errors.我一直在四处寻找，似乎找不出来。我的文件如下：

Gruntfile.js:

module.exports = function(grunt) {
            // 1. All configuration goes here 
            grunt.initConfig({
                pkg: grunt.file.readJSON('package.json'),
                concat: {
                    dist: {
                        src: [
                            'js/libs/*.js', // All JS in the libs folder
                            'js/controls.js', // This specific file
                        ],
                        dest: 'dist/built.js',
                    }
                },
                uglify: {
                    build: {
                        src: 'js/build/production.js',
                        dest: 'js/build/production.min.js',
                    }
                },
            });
            // 3. Where we tell Grunt we plan to use this plug-in.
            grunt.loadNpmTasks('grunt-contrib-concat');
            grunt.loadNpmTasks('grunt-contrib-uglify');
            // 4. Where we tell Grunt what to do when we type 'grunt' into the terminal.
            grunt.registerTask('default', ['concat, uglify']);
        };

package.json:

{
  "name": "grunt_libsass_example-project",
  "version": "0.1.0",
  "devDependencies": {
    "grunt": "~0.4.1",
    "grunt-contrib-concat": "^0.5.1",
    "grunt-contrib-uglify": "^0.9.1"
  }
}