无法在FirebaseListObservable上使用mergeMap？

Question

我尝试在FirebaseListObservable上使用mergeMap时出现以下错误：

ERROR TypeError: chats.mergeMap is not a function
    at MapSubscriber.project (tabs.ts:93)
    at MapSubscriber._next (map.js:77)
    at MapSubscriber.Subscriber.next (Subscriber.js:89)
    at Notification.observe (Notification.js:32)
    at QueueAction.ObserveOnSubscriber.dispatch (observeOn.js:50)
    at QueueAction.AsyncAction._execute (AsyncAction.js:111)
    at QueueAction.execute (QueueAction.js:33)
    at QueueScheduler.AsyncScheduler.flush (AsyncScheduler.js:36)
    at QueueAction.schedule (QueueAction.js:27)
    at QueueScheduler.Scheduler.schedule (Scheduler.js:43)

这是我的组件代码：

this.msgService.getUserChatList()
          .mergeMap( chats => chats.mergeMap( chat => {
                return this.msgService.getLastMessage(chat.$key)
                  .map( lastMess => {
                    console.log('lastMess ', lastMess);
                    return this.containsUnreadMessages(lastMess, chat.lastPresence);
                  });
                } )
          )
          .last()
          .subscribe( (results) => {
            console.log('finished ', results)
            // Set global unread var
            this.unread = this.anyUnread;
          } );

返回FirebaseListObservable的getUserChatList函数：

getUserChatList(currentUserUid?: string): any {
    let company = this.authService.getCurrentUserCompany();
    let uid;
    currentUserUid ? uid = currentUserUid : uid = this.authService.getCurrentUserUid();
    let path = `/${company}/users/${uid}/chats`;
    return this.af.list(path);
  }

看起来我们必须双重包装FirebaseListObservable Github feature request

知道我为什么会收到这个错误吗？