reactjs Cannot read property 'keys' of undefined
<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>