我在任何从firestore接收的类型的observable上面临切片操作错误 . 我正在尝试使用firestore数据自动完成角度材料 .

export class HomeComponent implements OnInit {

customerCtrl: FormControl;
filteredCustomer: Observable<any[]>;
customers_coll: AngularFirestoreCollection<any>;
customers: Observable<any[]>;

constructor(private afs: AngularFirestore,private _data: DataService) {
console.log("home");
this.customerCtrl = new FormControl();
this.customers_coll = 
this.afs.collection('USERS').doc(this._data.phone).collection("CUSTOMERS");
this.customers = this.customers_coll.valueChanges();

}

ngOnInit() {

this.filteredCustomer = this.customerCtrl.valueChanges
.pipe(
  startWith(''),
  map(customer => customer ? this.filterCustomer(customer) : 
  this.customers.slice())
  );
  }

  filterCustomer(name: string) {
  return this.customers.filter(customer =>
    customer.NAME.toLowerCase().indexOf(NAME.toLowerCase()) === 0);
  }
  }