cyclic dependency error when importing service into another service

问题

I'm trying to set default headers for my http requests using the DefaultRequestOptions class provided by Angular 2. Documentation can be found here: https://angular.io/docs/ts/latest/guide/server-communication.html#!#override-default-request-options

I want to add a default bearer token, which gets set in one of my services, but doing so gives me the following error in my browser console:

Unhandled Promise rejection: Provider parse errors: Cannot instantiate cyclic dependency! Http: in NgModule AppModule in ./AppModule ; Zone: ; Task: Promise.then ; Value:

Here is my default-request-options.service.ts file:

import { Injectable }                         from '@angular/core';
import { BaseRequestOptions, RequestOptions } from '@angular/http';
import { UserService }                        from './user.service';

@Injectable()
export class DefaultRequestOptions extends BaseRequestOptions {

  constructor(private userService: UserService) {
    super();

    // Set the default 'Content-Type' header
    this.headers.set('Content-Type', 'application/json');
    this.headers.set('Accept', 'application/json');
    this.headers.set('Authorization', 'Bearer ' + this.userService.idToken);
  }
}

export const requestOptionsProvider = { provide: RequestOptions, useClass: DefaultRequestOptions };

Here is the relevant code in my app.module.ts file:

import { requestOptionsProvider }         from './default-request-options.service';
import { UserService }                    from './user.service';

@NgModule({ 
    imports: [
        ...
    ],  
    declarations: [
        ...
    ],    
    providers: [        
        ...        
        requestOptionsProvider,        
        UserService
    ],
    bootstrap: [ AppComponent ]
})

What am I doing wrong?