Webpack-extract-text-webpack-plugin找不到模块

webpack.config.js

var ExtractTextPlugin = require("extract-text-webpack-plugin");

如果我只是实现插件，我会立即收到此错误：

module.js:339
    throw err;
    ^

Error: Cannot find module 'webpack/lib/ConcatSource'
    at Function.Module._resolveFilename (module.js:337:15)
    at Function.Module._load (module.js:287:25)
    at Module.require (module.js:366:17)
    at require (module.js:385:17)
    at Object.<anonymous> (/Users/lucamormile/Documents/Lavori/Webapps/React/webpack_test/node_modules/extract-text-webpack-plugin/index.js:5:20)
    at Module._compile (module.js:425:26)
    at Object.Module._extensions..js (module.js:432:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:311:12)
    at Module.require (module.js:366:17)

我忘了什么？