Angular2 NgFor仅支持绑定到Iterables,例如Arrays错误

我有一个模特：

export class CoreGoalmodel {
  constructor(

    public title: String,public image: String,){}
}

我的服务：

getGoals(): Observable<CoreGoalmodel[]> {

    let headers = new Headers({ 'Access-Control-Allow-Origin': '*' });
    let options = new requestOptions({ headers: headers });

    return this.http.get(this.base_url + 'coregoal',options)
    .map(this.extractData)
    .catch(this.handleError);
  }

  private extractData(res: ResponsE) {
    let body = res.json();
    return body.data || { };
  }

然后在我的组件中：

ngOnInit() {

    this.homeservice.getGoals()
    .subscribe(
                 goals => this.coreGoals = goals,error =>  this.errormessage = <any>error);

}

然后我在我的模板中将其绑定为：

<ul>
    <li *ngFor="let goal of coreGoals">
        {{goal.titlE}}
    </li>
</ul>

我从服务器的实际响应：

[{"coreGoalId":1,"title":"Core goal 1","infrastructure":"Sample Infrastructure","audience":"People","subGoals":null,"benefits":[{"benefitId":1,"what":"String","coreGoalId":1}],"effects":null,"steps":null,"images":[{"imagelId":1,"base64":"/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAYEBQYFBAYGBQYHBwYIChAKCgkJChQODwwQFxQYGBcU\nFhYaHSUfGhsjHBYWICwgIyYnKSopGR8tMC0oMCUoKSj/2wBDAQcHBwoIChMKChMoGhYaKCgoKCgo\nKCgoKCgoKCgoKCgoKCgoKCgoKCgoKCgoKCgoKCgoKCgoKCgoKCgoKCgoKCj/wgARCAIWAe4DASIA\nAhEBAxEB/8QAHAABAAIDAQEB"}]}]

这会引发我的错误无法找到’object’类型的不同支持对象'[object Object]’. NgFor仅支持绑定到诸如Arrays之类的Iterables.

我究竟做错了什么？我只想迭代coreGoals属性,并访问它的子项及其属性.

private extractData(res: ResponsE) {
  let body = res.json();
  return body.data || { }; // error
}

private extractData(res: ResponsE) {
  let body = res.json();
  return body || { }; // here
}