app.default is not a function

[rootlinux2]

I'm facing this error, I have the same webpack config as in your readme file.

 Error: HtmlWebpackPrerenderPlugin ERROR: Error rendering component.
  TypeError: app.default is not a function

My index.js

import React from 'react';
import ReactDOM from 'react-dom';
import ReactDOMServer from 'react-dom/server';
import MyComponent from '../../pages/index';

const App = () => <h1>Hello world!</h1>;

if (document.body) {
  const div = document.getElementById('root');
  if (div.hasChildNodes()) {
    ReactDOM.hydrate(<MyComponent />, div);
  } else {
    ReactDOM.render(<MyComponent />, div);
  }
}

export default () => ReactDOMServer.renderToString(<App />);