Cloud Function built by webpack

firebase.json

{
	"functions": {
		"source": "functions"
	},
	"hosting": {
		"public": "build",
		"ignore": ["firebase.json", "**/.*", "**/node_modules/**"],
		"rewrites": [
			{
				"source": "/api/hello",
        "function": "hello"
			}
		]
	}
}

index.js

// server/index.js
import * as functions from 'firebase-functions'

export const hello = functions.https.onRequest((req, res) => {
  res.send('hello').end()
})

package.json

{
	"private": true,
	"scripts": {
		"deploy:functions": "webpack --config server/webpack.config.js && cd functions && yarn install && firebase deploy --only functions"
	},
	"devDependencies": {
		"babel-loader": "^7.1.2",
		"generate-json-webpack-plugin": "^0.2.2",
		"webpack": "^3.8.1"
	},
	"dependencies": {
		"firebase-admin": "^5.5.1",
		"firebase-functions": "^0.7.3"
	}
}

webpack.config.js

// server/webpack.config.js
const pkg = require('../package')
const GenerateJsonPlugin = require('generate-json-webpack-plugin')

const externals = ['firebase-admin', 'firebase-functions']

const genPackage = () => ({
	name: 'functions',
	private: true,
	main: 'index.js',
	license: 'MIT',
	dependencies: externals.reduce(
		(acc, name) =>
			Object.assign({}, acc, {
				[name]: pkg.dependencies[name] || pkg.devDependencies[name]
			}),
		{}
	)
})

module.exports = {
	entry: [__dirname + '/index.js'],
	output: {
		path: __dirname + '/../functions',
		filename: 'index.js',
		libraryTarget: 'commonjs'
	},
	module: {
		rules: [
			{
				test: /\.js$/,
				use: 'babel-loader',
				include: __dirname + '/src'
			}
		]
	},
	externals: externals.reduce(
		(acc, name) => Object.assign({}, acc, { [name]: true }),
		{}
	),
	plugins: [new GenerateJsonPlugin('package.json', genPackage())]
}

i face the following error: [webpack-cli] TypeError: Cannot read property 'PROCESS_ASSETS_STAGE_ADDITIONAL' of undefined