Angular / AWS Cognito User Registration Error - 'Unknown error, the response body from fetch is: undefined'

After receiving this response when attempting to register new users in my Angular 7 application, I've seen this error reported before for Cognito authentication, usually also regarding user registration and confirmation.

However, I'm yet to see any suggestions that may work within an Angular application, and any solutions previously-suggested have failed, including adding the 'node-fetch' module.

I receive the error on attempting both the userPool.signUp() and cognitoUser.confirmRegistration() methods, even though each work successfully in registering/confirming the user.

register.component.ts:

import { Component, OnInit } from '@angular/core';
import { Router } from '@angular/router';
import fetch from 'node-fetch';
import * as AmazonCognitoIdentity from 'amazon-cognito-identity-js';
import { AuthService } from '@app/services/auth/auth.service';

@Component({
  selector: 'app-register',
  templateUrl: './register.component.html',
  styleUrls: ['./register.component.scss']
})

export class RegisterComponent implements OnInit {

  fetch;
  attributes: AmazonCognitoIdentity.CognitoUserAttribute[];

  email: string;
  password: string;
  confirmationCode: string;

  constructor(private auth: AuthService, public router: Router) {
    this.attributes = [];
    this.fetch = fetch;
  }

  ngOnInit() {

  }

  register() {
    this.attributes.push(new AmazonCognitoIdentity.CognitoUserAttribute({
        Name: 'email',
        Value: this.email
    }))

    this.auth.userPool.signUp(this.email, this.password, this.attributes, null, function(err, res) {
        if (err) {
            alert(err.message || JSON.stringify(err));
            return;
        } else {
            console.log('Username is' + res.getUsername());
        }
    })
  }

  confirmRegistration() {
    const userData = {
        Username: this.email,
        Pool: this.auth.userPool
    }

    let cognitoUser = new AmazonCognitoIdentity.CognitoUser(userData);

    cognitoUser.confirmRegistration(this.confirmationCode, true, function(err, res) {
        if (err) {
            alert(`Error confirming registration: ${err.message}`);
            console.error('Error confirming registration:', err);
        } else {
            console.log('User registration confirmed', res);
            this.auth.user = cognitoUser;
        }
    })
  }

  resendConfirmationCode() {
    const userData = {
        Username: this.email,
        Pool: this.auth.userPool
    }

    let cognitoUser = new AmazonCognitoIdentity.CognitoUser(userData);

    cognitoUser.resendConfirmationCode(function(err, res) {
        if (err) {
            alert(err.message || JSON.stringify(err));
            console.log('Error sending confirmation code:', err);
        } else {
            console.log(`Re-sent confirmation code to email address '${this.email}'`);
        }
    })
  }

}