angular2服务基类继承-为什么“this”为空?

我试图使用继承为我的服务创建一个通用的错误处理程序,但由于某种原因,当它到达错误处理程序本身时,“this”看起来总是空的,我不知道为什么。我可以很好地进入错误处理程序,但我总是得到:

异常:未捕获(承诺中):类型错误:无法读取属性 空的“http”

你知道我错过了什么/做错了什么吗?不确定“this”怎么可能为空?

这是我的整个服务基类:

import { Injectable } from '@angular/core';
import { Http } from '@angular/http';

@Injectable()
export class HttpServiceBase {

    constructor(public http: Http) {
        console.log('http', this.http); //just do this to prove that it is there - it is!
    }

    handleError(error: any): Promise<any> {
        console.error('Application Error', error); //this logs fine

        // TypeError: Cannot read property 'http' of null
        this.http.get('/Account/IsLoggedIn')
            .map(response => console.log('RESPONSE: ', response));

        return Promise.reject(error.message || error);
    }
}

这是我的继承类:

import 'rxjs/add/operator/toPromise';
import { Injectable } from '@angular/core';
import { Headers, Http } from '@angular/http';
import { HttpServiceBase } from './http-service.base';
import { Hero } from './hero';

@Injectable()
export class HeroService extends HttpServiceBase {

    private headers = new Headers({ 'Content-Type': 'application/json' });
    private heroesUrl = 'http://localhost:57569/Home/Heroes';

    constructor(http: Http) { super(http); }

    getHeroes(): Promise<Hero[]> {
        console.log('getting heroes');

        return this.http.get(this.heroesUrl + '-force-error') //so it will error out
            .toPromise()
            .then(response => response.json() as Hero[] )
            .catch(this.handleError);
    }
}