electron-builder: Application entry can't be found

Hey,all I followed the two package.json structure in the document, but when I ran npm run build, it reported an error that Application entry file couldn't be found in package.Seems like a wrong configuration

the package.json in the root of the project is:

  ...
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1",
    "postinstall": "install-app-deps",
    "clean": "rimraf dist",
    "build": "build"
  },
  "build": {
    "app-bundle-id": "com.smartstudy.toefl",
    "app-category-type": "public.app-category.education"
  },
  ...

and the structure of app directory is: the package.json in app directory is:

{
  "name": "toefl",
  "version": "0.0.1",
  "description": "this is toefl-pc",
  "main": "./index.js",
  "private": true,
  "author": "caibirdme",
  "license": "ISC",
  "dependencies": {
    "request": "^2.71.0",
    "sequelize": "^3.21.0",
    "sqlite3": "^3.1.3"
  }
}

version: node 5.10.0 npm 3.8.3 electron 0.37.5 electron-builder 3.13.1