UglifyJsPlugin is preventing generation of sourcemaps in Webpack 2

Readme.md

Install & run:

npm i webpack@2.1.0-beta.20
./node_modules/.bin/webpack

Output with UglifyJsPlugin

           Asset     Size  Chunks             Chunk Names
    index.min.js  2.59 kB       0  [emitted]  main

Output without UglifyJsPlugin

           Asset     Size  Chunks             Chunk Names
    index.min.js  2.59 kB       0  [emitted]  main
index.min.js.map  2.54 kB       0  [emitted]  main

UglifyJsPlugin is preventing generation of sourcemaps in Webpack 2

index.js

// code

webpack.config.js

var webpack = require('webpack');

module.exports = {
    context: __dirname,
    devtool: 'source-map',
    entry: './index.js',
    output: {
        filename: 'index.min.js'
    },
    plugins: [
        new webpack.optimize.UglifyJsPlugin()
    ]
}