Trying out the example on page 9:

​ curl https://activity-atk.herokuapp.com/list

returns:

<!DOCTYPE html>
        <html>
          <head>
                <meta name="viewport" content="width=device-width, initial-scale=1">
                <meta charset="utf-8">
                <title>Application Error</title>
                <style media="screen">
                  html,body,iframe {
                        margin: 0;
                        padding: 0;
                  }
                  html,body {
                        height: 100%;
                        overflow: hidden;
                  }
                  iframe {
                        width: 100%;
                        height: 100%;
                        border: 0;
                  }
                </style>
          </head>
          <body>
                <iframe src="//www.herokucdn.com/error-pages/application-error.html"></iframe>
          </body>
        </html

Any ideas?