title: PouchDB error "Uncaught (in promise) TypeError: doc is undefined"
body: <p>I am always get this error, but only with Firefox – with Chrome it works.</p>
  <p>I only have copied the demo code from the PouchDB website.</p>
  <pre><code>var db = new PouchDB('todos');
  var remoteCouch = false;
  
  db.changes({
    since: 'now',
    live: true
  }).on('change', showTodos);
  
  function addTodo(text) {
    var todo = {
      _id: new Date().toISOString(),
      title: text,
      completed: false
    };
    db.put(todo, function callback(err, result) {
      if (!err) {
        console.log('Successfully posted a todo!');
      }
    });
  }
  
  function showTodos() {
    db.allDocs({include_docs: true, descending: true}, function(err, doc) {
      redrawTodosUI(doc.rows);
    });
  }
  </code></pre>
  <p>Have anyone an idea why?</p>
  <p>Thank you in advance.</p>