ReactJs browser Cannot read property 'keys' of undefined
<p><strong>HTML code:</strong></p>

<pre><code>&lt;div id="content"&gt;&lt;/div&gt;

&lt;script src="build/react.min.js"&gt;&lt;/script&gt;
&lt;script src="build/react-dom.min.js"&gt;&lt;/script&gt;
&lt;script src="https://cdn.bootcss.com/babel-core/6.1.19/browser.min.js"&gt;&lt;/script&gt;
&lt;script src="ex1.jsx" type="text/babel"&gt;&lt;/script&gt;
</code></pre>

<p><strong>JSX code:</strong></p>

<pre><code>// create class
var HelloWord = React.createClass({
    render: function () {
        return (
            &lt;div&gt;
                &lt;p&gt;Hello Word!&lt;/p&gt;
            &lt;/div&gt;
        );
    }
});

// show content
ReactDOM.render(
    &lt;HelloWord&gt;&lt;/HelloWord&gt;, document.getElementById('content')
);
</code></pre>

<p>Console message after run:</p>

<blockquote>
  <p>Uncaught TypeError: Cannot read property 'keys' of undefined</p>
</blockquote>

<p>Why?</p>