reveal.js: Slide Notes timer doesn't work

When I open the Slide Notes browser page (pressing s) the timer doesn’t start. I’m using Chromium and I’m getting this error in the console

Uncaught ReferenceError: Reveal is not defined
    at getTimings (notes.html:484)
    at setupTimer (notes.html:541)
    at handleConnectMessage (notes.html:387)
    at notes.html:354

this doesn’t happen in the standard view. I’m using the latest git version with Chromium 59. I’m initializing the presentation like this:

    <script src="../reveal.js/lib/js/head.min.js"></script>
    <script src="../reveal.js/js/reveal.js"></script>
    <script>
      Reveal.initialize({
          dependencies: [
            // Interpret Markdown in <section> elements
            { src: '../reveal.js/plugin/markdown/marked.js', condition: function() { return !!document.querySelector( '[data-markdown]' ); } },
            { src: '../reveal.js/plugin/markdown/markdown.js', condition: function() { return !!document.querySelector( '[data-markdown]' ); } },
            { src: '../reveal.js/plugin/notes/notes.js', async: true }
          ],
          slideNumber: false
      });
    </script>