类型'Promise <string []>'不能分配给类型'string []'

出现以下错误：类型'Promise'不能分配给类型'string []'。类型“ Promise”中缺少属性“ includes”。

当我将Promise强制键入“ string []”下面的代码时，

组件：app.dashboard.ts

import {Component} from '@angular/core';
import { MemberService } from "./app.service";
@Component({
selector:'app-root',
templateUrl:'./app.dashboard.html',
providers:[MemberService]
          })

export class AppDashboard{
  title='Dashboard'
  constructor(private memberService: MemberService) { }

  public doughnutChartLabels:string[] = 
    this.memberService.getmemberheader();//error occurred here
  }
}

服务：app.service.ts

import { Injectable } from '@angular/core';
import { Member } from './Member';
import { Http, Response, Headers, RequestOptions, URLSearchParams } from'@angular/http';
import 'rxjs/add/operator/map';
import 'rxjs/add/operator/toPromise';

@Injectable()
export class MemberService
{
  constructor(private http: Http) {
  }

  private getHeaders(){
    // I included these headers because otherwise FireFox
    // will request text/html instead of application/json
    let headers = new Headers();
    headers.append('Accept', 'application/json');
    return headers;
  }

  getmemberheader(): Promise<string[]> {
    return this.http
        .get(`/ReportService/MemberDatabaseCountryname`, {headers: this.getHeaders()})
        .toPromise()
        .then(this.extractData)
        .catch(this.handleError);
  }  

  private extractData(res: Response) {
    let body = res.json();

    return body || {};
  }
  private handleError(error: any): Promise<any> {
    console.error('An error occurred', error);
    return Promise.reject(error.message || error);
  }
}