React runs the browser and reports an error [A may have only one child element]

The error message

Uncaught Invariant Violation: A <Router> may have only one child element
    at invariant (http://localhost:8111/bundle.js:1071:15)
    at Router.componentWillMount (http://localhost:8111/bundle.js:2413:54)
    at callComponentWillMount (http://localhost:8111/bundle.js:27042:14)
    at mountClassInstance (http://localhost:8111/bundle.js:27135:5)
    at updateClassComponent (http://localhost:8111/bundle.js:30309:5)
    at beginWork (http://localhost:8111/bundle.js:31265:16)
    at performUnitOfWork (http://localhost:8111/bundle.js:34933:12)
    at workLoop (http://localhost:8111/bundle.js:34973:24)
    at renderRoot (http://localhost:8111/bundle.js:35056:7)
    at performWorkOnRoot (http://localhost:8111/bundle.js:35963:7)

The solution

The original demo 

ReactDom.render(
    <BrowserRouter>
	    <Nav />
	    {getRouter()}
    </BrowserRouter>,
    document.getElementById('app')
)

Resolved Demo (if there are multiple components in The BrowserRouter, add a layer of div to the BrowserRouter) 

ReactDom.render(
    <BrowserRouter>
        <div>
            <Nav />
            {getRouter()}
        </div>
    </BrowserRouter>,
    document.getElementById('app')
)

As for the reasons, I haven’t studied them in detail yet. If you know some friends, you can communicate with them, or explain them in the comments below. Thank you.