(extract-text-webpack-plugin) error [1] "loader" must be a string

After I installed the extract-text-webpack-plugin it started to return me the error erro [1] "loader" must be a string

Follow my webpack configuration

'use strict'

const path = require('path')
const webpack = require('webpack')
const validate = require('webpack-validator')

const HtmlPlugin = require('html-webpack-plugin')
const ExtractTextPlugin = require('extract-text-webpack-plugin')

module.exports = validate({

  devtool: 'source-map',

  entry: [
    'react-hot-loader/patch',
    'webpack-dev-server/client?http://localhost:3000',
    'webpack/hot/only-dev-server',
    path.join(__dirname, 'src', 'index')
  ],

  output: {
    path: path.join(__dirname, 'dist'),
    filename: '[name]-[hash].js',
    publicPath: ''
  },

  plugins: [
    new webpack.HotModuleReplacementPlugin(),
    new ExtractTextPlugin('[name]-[hash].css'),
    new HtmlPlugin({
      title: 'GitHub app',
      template: path.join(__dirname, 'src', 'html', 'template.html')
    })
  ],

  module: {
    preLoaders: [{
      test: /\.js$/,
      exclude: /node_modules/,
      loader: 'standard'
    }],

    loaders: [{
      test: /\.js$/,
      exclude: /node_modules/,
      include: /src/,
      loader: 'babel'
    }, {
      test: /\.css$/,
      exclude: /node_modules/,
      include: /src/,
      loader: ExtractTextPlugin.extract('style', 'css')
    }]
  }
})

I'll leave the repository link