angularjs – TypeError：无法读取[null]中未定义的属性’post’

EXCEPTION: TypeError: CAnnot read property 'post' of undefined in [null]
ORIGINAL EXCEPTION: TypeError: CAnnot read property 'post' of undefined
ORIGINAL STACKTRACE:
TypeError: CAnnot read property 'post' of undefined
at LisTingservice.getLisTings (http://localhost:3000/app/lisTing.service.js:30:30)
at LisTingsComponent.getLisTings (http://localhost:3000/app/lisTings.component.js:45:41)
at LisTingsComponent.ngOnInit (http://localhost:3000/app/lisTings.component.js:41:26)
at AbstractChangeDetector.ChangeDetector_HostLisTingsComponent_0.detectChangesInRecordsInternal (eval at <anonymous> (http://localhost:3000/node_modules/angular2/bundles/angular2.dev.js:10897:14),<anonymous>:22:99)
at AbstractChangeDetector.detectChangesInRecords (http://localhost:3000/node_modules/angular2/bundles/angular2.dev.js:8824:14)
at AbstractChangeDetector.runDetectChanges (http://localhost:3000/node_modules/angular2/bundles/angular2.dev.js:8807:12)
at AbstractChangeDetector._detectChangesContentChildren (http://localhost:3000/node_modules/angular2/bundles/angular2.dev.js:8871:14)
at AbstractChangeDetector.runDetectChanges (http://localhost:3000/node_modules/angular2/bundles/angular2.dev.js:8808:12)
at AbstractChangeDetector._detectChangesInViewChildren (http://localhost:3000/node_modules/angular2/bundles/angular2.dev.js:8877:14)
at AbstractChangeDetector.runDetectChanges (http://localhost:3000/node_modules/angular2/bundles/angular2.dev.js:8811:12)

我的LisTingservice.ts看起来像这样：

import {Injectable,Injector} from 'angular2/core';
import {http_PROVIDERS,http,Headers} from 'angular2/http';
import {LisTing} from './lisTing';
import 'rxjs/add/operator/map';
@Injectable()
export class LisTingservice {
 http: http;
 lisTings: Array<LisTing>;
 getLisTings() {
    var headers = new Headers();
    headers.append('Content-Type','application/json');
    this.http.post('http://144.376.29.134:and-so-on',JSON.Stringify(
    {"id":1,"title": "title","description": "Проводите","discount": "21%","imageUrl": "http://lorempixel.com/400/200/sports","tags": [{"tag": "Еда"}]
    }),{headers:headers}
    ).map(res => res.json())
    .subscribe((res:Array<LisTing>) => this.lisTings = res);
    return Promise.resolve(this.lisTings);
 }
 getLisTing(id: number) {
    return Promise.resolve(this.lisTings)
    .then(lisTings => lisTings.filter(l => l.id === id)[0]);
 }
}

使用LisTingservice的LisTingsComponent如下所示：

import {Component,OnInit} from 'angular2/core';
import {http_PROVIDERS,Headers} from 'angular2/http';
import {LisTing} from './lisTing';
import {LisTingservicE} from './lisTing.service';
import 'rxjs/add/operator/map';
@Component({
  ....
})
export class LisTingsComponent implements OnInit {
 lisTings: Array<LisTing>;
 constructor(private lisTingservice:LisTingservice,private _router: Router) {}
 ngOnInit() {
  this.getLisTings();
 }
 getLisTings() {
  this.lisTingservice.getLisTings().then(lisTings => this.lisTings = lisTings);
 }
 getLisTing(id: number) {
  return Promise.resolve(this.lisTings)
  .then(lisTings => lisTings.filter(l => l.id === id)[0]);
 }
 gotoDetail(lisTing: LisTing) {
  this._router.navigate(['LisTingDetail',{ id: lisTing.id }]);
 }
}

这有什么问题？