我正在使用firebase@5.0.4＆angularfire2@5.0.0-rc.11在angular 6上创建聊天应用程序，但是我遇到了一个错误，无法理解它的来源。请任何人可以帮助。 这是错误日志。
ERROR TypeError: rxjs_operators__WEBPACK_IMPORTED_MODULE_4__.switchMap.call(...).subscribe is not a function
at FirebaseListObservable._subscribe (firebase_list_factory.js:75)
at FirebaseListObservable.push../node_modules/rxjs/_esm5/internal/Observable.js.Observable._trySubscribe (Observable.js:42)
at FirebaseListObservable.push../node_modules/rxjs/_esm5/internal/Observable.js.Observable.subscribe (Observable.js:28)
at ObservableStrategy.push../node_modules/@angular/common/fesm5/common.js.ObservableStrategy.createSubscription (common.js:4466)
at AsyncPipe.push../node_modules/@angular/common/fesm5/common.js.AsyncPipe._subscribe (common.js:4546)
at AsyncPipe.push../node_modules/@angular/common/fesm5/common.js.AsyncPipe.transform (common.js:4527)
at Object.eval [as updateDirectives] (FeedComponent.html:2)
at Object.debugUpdateDirectives [as updateDirectives] (core.js:10756)
at checkAndUpdateView (core.js:10153)
at callViewAction (core.js:10394)