create-react-app: The script has an unsupported MIME type ('text/html')

Hi I’ve written a small project with create-react-app, it runs in the browser ‘http://localhost:3000/’ with ‘npm start’ but i’m seeing these two errors in the console:

The script has an unsupported MIME type ('text/html').  :3000/service-worker.js:1
GET http://localhost:3000/service-worker.js net::ERR_INSECURE_RESPONSE  :3000/service-worker.js:1

this is my package.json

{
  "name": "my-app",
  "version": "0.1.0",
  "private": true,
  "devDependencies": {
    "react-scripts": "0.4.1"
  },
  "dependencies": {
    "react": "^15.3.1",
    "react-dom": "^15.3.1",
    "react-redux": "^4.4.5",
    "redux": "^3.6.0"
  },
  "scripts": {
    "start": "react-scripts start",
    "build": "react-scripts build",
    "test": "react-scripts test --env=jsdom",
    "eject": "react-scripts eject"
  },
  "eslintConfig": {
    "extends": "./node_modules/react-scripts/config/eslint.js"
  },
  "standard": {
    "globals": [
      "describe",
      "context",
      "before",
      "beforeEach",
      "after",
      "afterEach",
      "it",
      "expect"
    ]
  }
}