title: reactjs Cannot read property 'keys' of undefined
body: <p>I am learning reactjs through a tutorial and ran into this error. That says "Cannot read property 'keys' of undefined" My code is very minimal so I assume that it has to do with the structure of the language. Does anyone know the problem and a possible solution?</p>
  
  <pre><code>   &lt;!DOCTYPE html&gt;
  
  &lt;html&gt;
  &lt;head&gt;
  
      &lt;script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.0.1/react.min.js"&gt;&lt;/script&gt;
      &lt;script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.0.1/react-dom.min.js"&gt;&lt;/script&gt;
      &lt;script src="https://cdnjs.cloudflare.com/ajax/libs/babel-core/6.1.19/browser.min.js"&gt;&lt;/script&gt;
      &lt;title&gt;ReactJs&lt;/title&gt;
  &lt;/head&gt;
  &lt;body&gt;
      &lt;div id="app"&gt;&lt;/div&gt;
  
      &lt;script type="text/babel"&gt;
          var HelloWorld = ReactDOM.createClass({
          render: function() {
          return
          &lt;div&gt;
              &lt;h1&gt;Hello World&lt;/h1&gt;
              &lt;p&gt;This is some text&gt;&lt;/p&gt;
          &lt;/div&gt;
          }
          });
          ReactDOM.render(
          &lt;HelloWorld /&gt;, document.getElementById('app'));
      &lt;/script&gt;
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>