starting a react app raises "A may have only one child element" at "src/index.js"

Question

Below is my index.js file:

import React from 'react';
import ReactDOM from 'react-dom';
import { Provider } from 'react-redux';
import store from './store';
import App from './containers/App';
import './styles/bootstrap.css';
import './styles/font-awesome.css';
import './styles/index.css';


ReactDOM.render(

 <Provider store={store}>
    <App />
 </Provider>

 , document.getElementById('root')
);

What is causing the error in the title?