combineLatest with realtime database

combineLatest.ts

// rxjs 5.4
// angularfire2-4.0.0-rc.1

import { Injectable } from '@angular/core';
import { AngularFireDatabase } from 'angularfire2/database';

import 'rxjs/add/observable/combineLatest';
import {Observable} from 'rxjs/Observable';

@Injectable()
export class Service {
  constructor( private db: AngularFireDatabase ) {}
    
  subscribe(id: string): Observable<any> {
    // source1$ and source2$ are continuously emitting data over time
    const source1$ = this.db.object(`path1/${id}`);
    const source2$ = this.db.object(`path2/${id}`);
    return Observable.combineLatest(
      source1$,
      source2$
    )
      .map([data1, data2] => logicToCombineData(data1, data2));
  }
}

I tried this but the map never fires?