title: Uncaught Error: Element type is invalid: expected a string
body: <p>I've tried <a href="http://codepen.io/gaearon/pen/VKQwEo?editors=0010" rel="nofollow noreferrer">http://codepen.io/gaearon/pen/VKQwEo?editors=0010</a> from React's documentation it works fine.</p>
  
  <p>I want to make a button using material-ui.
  What is wrong with this code?</p>
  
  <pre><code>    import * as React from "react";   
      import * as ReactDOM from "react-dom";
      import {Router, Route, IndexRoute} from "react-router";
  
      const FlatButton = require('material-ui/FlatButton');
  
  
      function Test(props){
      return(
      &lt;div className="test"&gt;
      aaa
      &lt;FlatButton label="Default" /&gt;
      &lt;/div&gt;
      );
      }
  
      ReactDOM.render(
      &lt;Test
  
      /&gt;
      , document.getElementById('root')
      );
  </code></pre>
  
  <p>Errors:</p>
  
  <hr>
  
  <pre><code>invariant.js:39 Uncaught Error: Element type is invalid: expected a string (for built-in components) or a class/function (for composite components) but got: object. Check the render method of `Test
  
  warning.js:45 Warning: React.createElement: type should not be null, undefined, boolean, or number. It should be a string (for DOM elements) or a ReactClass (for composite components). Check the render method of `Test`
  </code></pre>
  
  <hr>