React.Children.only expected to get one child React element error

I am getting "React.Children.only expected to receive one React child". from the following code.

I created a new react-app using create-react-app and setting both react and react-reduce and created a simple reducer and created a repository with it.

import React from "react";
import ReactDOM from "react-dom";
import { createStore, combineReducers } from "redux";
import { Provider } from "react-redux";

import logo from './logo.svg';
import './App.css';

class App extends React.Component {
  render() {
    return (
      <div className="App">
        <div className="App-header">
          <img src={logo} className="App-logo" alt="logo" />
          <h2>Welcome to React</h2>
        </div>
        <p className="App-intro">
          To get started, edit <code>src/App.js</code> and save to reload.
        </p>
      </div>
    );
  }
}


function productReducer(state = [], action) {
  return state;
}

const store = createStore(
  productReducer
);

ReactDOM.render((<Provider store={store}><App /> </Provider>), document.getElementById('root'));