502 server error when deploying React app to Google App Engine

Google App Engine newbie here.

I've deployed my React app to google, but when I try to preview it I get a 502 error:

Failed to load resource: the server responded with a status of 502 ()

app.yaml:

env: flex
runtime: nodejs

package.json:

{
  "name": "react-boilerplate",
  "version": "1.0.0",
  "description": "Minimal boilerplate for react",
  "main": "index.js",
  "scripts": {
    "start": "node server.js",
    "bundle": "./node_modules/.bin/webpack --config webpack.config.js",
    "prestart": "npm run bundle"
  },
  "author": "",
  "license": "ISC",
  "babel": {
    "presets": [
      "es2015",
      "react",
      "stage-2"
    ]
  },
  "engines": {
    "node": "6.11.0",
    "npm": "3.10.10"
  },
  "devDependencies": {
    "babel-core": "^6.24.1",
    "babel-loader": "^7.0.0",
    "babel-preset-es2015": "^6.24.1",
    "babel-preset-react": "^6.24.1",
    "babel-preset-stage-2": "^6.24.1",
    "css-loader": "^0.28.0",
    "node-sass": "^4.5.2",
    "sass-loader": "^6.0.5",
    "style-loader": "^0.16.1",
    "webpack": "^3.0.0",
    "webpack-dev-server": "^2.4.5"
  },
  "dependencies": {
    "axios": "^0.16.2",
    "d3": "^4.9.1",
    "express": "^4.15.3",
    "pug": "^2.0.0-rc.2",
    "react": "^15.5.4",
    "react-dom": "^15.5.4",
    "react-redux": "^5.0.5",
    "redux": "^3.7.1",
    "redux-thunk": "^2.2.0",
    "webpack": "^3.3.0"
  }
}

webpack.config.js:

var webpack = require('webpack');

module.exports = {
  entry: [
    './src/index.js'
  ],
  module: {
    loaders: [
      {
        test: /\.jsx?$/,
        exclude: /node_modules/,
        loader: 'babel-loader'
      },
      {
        test: [/\.css$/, /\.scss$/],
        exclude: /node_modules/,
        loaders: ['style-loader', 'css-loader', 'sass-loader']
      }
    ]
  },
  resolve: {
    extensions: ['*', '.js', '.jsx']
  },
  output: {
    path: __dirname + '/dist',
    publicPath: '/',
    filename: 'bundle.js'
  },
  devServer: {
    contentBase: './dist',
    historyApiFallback: true
  },
  plugins: [
    new webpack.DefinePlugin({
      'process.env.NODE_ENV': JSON.stringify(process.env.NODE_ENV)
    })
  ]
}

Running out of ideas. Any help would be awesome!