ERROR TypeError: res.json is not a function [duplicate]

I cannot understand what I am doing wrong, and why this error "res.json is not a function"

   import { Injectable } from '@angular/core';
import { HttpClient, HttpHeaders } from '@angular/common/http';
import { map } from 'rxjs/operators';

@Injectable({
  providedIn: 'root'
})
export class AuthService {

  constructor(private http: HttpClient) { }

  registerUser(user){
    let headers = new HttpHeaders();
    headers.append('Content-Type', 'application/json')
    return this.http.post(
      'http://localhost:3000/account/reg',
      user, 
      {headers: headers}).pipe(map((res: any) => res.json()));
  }
}