Angular 4/5 HttpClient: Argument of type string is not assignable to ‘body’

The Angular docs say:

The response body doesn't return all the data you may need. Sometimes
servers return special headers or status codes to indicate certain
conditions, and inspecting those can be necessary. To do this, you can
tell HttpClient you want the full response instead of just the body
with the observe option:

http
  .get<MyJsonData>('/data.json', {observe: 'response'})
  .subscribe(resp => {
    // Here, resp is of type HttpResponse<MyJsonData>.
    // You can inspect its headers:
    console.log(resp.headers.get('X-Custom-Header'));
    // And access the body directly, which is typed as MyJsonData as requested.
    console.log(resp.body.someField);
  });

But when I try that, I get a compilation time error (no runtime errors though, works as expected):

error TS2345: Argument of type '{ headers: HttpHeaders; observe: string; }' is not assignable to parameter of type '{ headers?: HttpHeaders | { [header: string]: string | string[]; }; observe?: "body"; params?: Ht…'.
Types of property 'observe' are incompatible.
Type 'string' is not assignable to type '"body"'.

Why? I use "@angular/http": "^5.1.0"

Here is my version of the code:

  login(credentials: Credentials): Observable<any> {
    const options = {
      headers: new HttpHeaders({'Content-Type': 'application/json'}),
      observe: 'response'
    };
    return this.httpClient.post<any>(`${environment.USER_SERVICE_BASE_URL}`,
      {'username': credentials.username, 'password': credentials.password}, options)
      .map((res) => ...