How to use less-plugin-clean-css with grunt-contrib-less?

I'm trying to use less-plugin-clean-css with grunt-contrib-less but I can't seem to get it to work. It seems grunt config can't find or install the plugin. This is for the bootstrap fork I support.

I have updated Gruntfile.js with

grunt.initConfig({
...
less: {
  compileCore: {
    options: {
      strictMath: true,
      sourceMap: true,
      outputSourceFiles: true,
   +  plugins: [
   +    (new require('less-plugin-clean-css')({
   +      "advanced": true,
   +      "compatibility": "ie8"
   +    }))
   +   ],
      sourceMapURL: '<%= pkg.name %>.css.map',
      sourceMapFilename: 'dist/css/<%= pkg.name %>.css.map'
    },
    src: 'less/bootstrap.less',
    dest: 'dist/css/<%= pkg.name %>.css'
  ...
  }
},

      

        
        
        
      

    

Plugin installed:

npm install less-plugin-clean-css --save-dev

      

        
        
        
      

    

Run grunt

and get this error:

Running "less:compileCore" (less) task
>> TypeError: Cannot read property 'install' of undefined

      

        
        
        
      

    

What am I missing?