title: FIREBASE WARNING: TypeError: Cannot read property 'then' of undefined
body: <p>According to the documentation, most of the firebase-database verbs return promises. Yet I cannot call <code>.then()</code> on <code>.once()</code> or <code>.remove()</code> or <code>.set()</code></p>
  
  <p>This is the version I am using: </p>
  
  <p>4.12.0</p>
  
  <p>Here is the error</p>
  
  <pre><code> FIREBASE WARNING: Exception was thrown by user callback. TypeError: Cannot read property 'then' of undefined
  </code></pre>
  
  <p>Here is the specific example:</p>
  
  <pre><code>   collab_ref = new Firebase(FBURL + 'collaboration_songs').child(collaboration_id);
     return collab_ref.remove().then(function(){
            return cb();
          });
  </code></pre>