当我运行我的简单程序时

索引.js

function test(state = []) {
    return state
}

const store = createStore(test);

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

应用程序js

const App = () => {
   return ( <h1 > Yo < /h1>)
}

export default App;

我得到的是这样的：

Error: React.Children.only expected to receive a single React element child.
    at invariant (invariant.js:44)
    at Object.onlyChild [as only] (onlyChild.js:33)
    at Provider.render (Provider.js:49)
    at ReactCompositeComponent.js:795
    at measureLifeCyclePerf (ReactCompositeComponent.js:75)

我做错了什么？当我删除提供程序渲染应用程序