How to set Headers using append or set does not work? in Angular 2

I am using Angular 2.4.8. I want to append custom headers to each request. I am using BaseRequestOptions to define custom headers and adding it to app providers. Following is the code.

import { BaseRequestOptions, Headers } from '@angular/http';
import { Injectable } from '@angular/core';

@Injectable()
export class AppBaseRequestOptions extends BaseRequestOptions {
public merge(headers: Headers) {
headers.append('Content-type', 'application/json');
headers.append('My-Custom-Header', 'My-Custom-Header-Value');
return super.merge(headers);
}
}


Provider is as follows:

providers: [
{ provide: BaseRequestOptions, useClass: AppBaseRequestOptions }
]


I have tried headers.append and headers.set but both throws

Cannot read property 'append' of null
Cannot read property 'set' of null

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