Error: InvalidPipeArgument: '' for pipe 'AsyncPipe' code example

Example: InvalidPipeArgument: '[object Object]' for pipe 'AsyncPipe'

export class MoviesComponent implements OnInit {
  movies: any[];

  constructor(private moviesDb: MoviesService) { }

  ngOnInit() {
    this.moviesDb.get().subscribe((snaps) => {
       this.movies = snaps;
   });
 }
}