Additional component in <MuiThemeProvider /> results in blank page w/ no error messages

Wow very strange but I managed to get it working by simply adding a <div>

app.js

const App = () => {
  return (
    <MuiThemeProvider muiTheme={getMuiTheme()}>
      <div>
        <Header />
        <NewPost />
      </div>
    </MuiThemeProvider>
  );  
}
Meteor.startup(() => {
  ReactDOM.render(<App />, document.querySelector('.render-target'));
});

I would really appreciate if anyone could explain why adding a div makes this all work. Thank you!