How to fix "Error: Token is empty. Please check your backend response." in Ember.js and Django REST Framework

I'm attempting to implement authentication in Ember.js using JSON Web Tokens generated from Django REST Framework.

Django 2.0.13 Django REST Framework 3.9.4 Django REST Simple JWT 4.3.0

I followed the example from this repository: https://github.com/jpadilla/ember-simple-auth-token

I have plugins for DRF that render and parse according to the JSON API standard, disabling them doesn't have any effect.

# urls.py
from rest_framework_simplejwt.views import TokenObtainPairView, TokenRefreshView
path("api/token/", TokenObtainPairView.as_view(), name='token_obtain_pair'),
    path('api/token/refresh/', TokenRefreshView.as_view(), name='token_refresh'),

// config/environment.js
  ENV['ember-simple-auth-token'] = {
    refreshAccessTokens: true,
    refreshLeeway: 300, // refresh 5 minutes (300 seconds) before expiration
    serverTokenEndpoint: 'http://localhost:8000/api/token/',
    serverTokenRefreshEndpoint: 'http://localhost:8000/api/token/refresh/',
    tokenPropertyName: 'access',
  };

// adapters/application.js
import DS from 'ember-data';
import TokenAuthorizerMixin from 'ember-simple-auth-token/mixins/token-authorizer';

export default DS.JSONAPIAdapter.extend(TokenAuthorizerMixin, {
  host: 'http://localhost:8000',
  namespace: 'api',
});

// controllers/u/login.js
import Controller from '@ember/controller';
import { inject } from '@ember/service';

export default Controller.extend({
  session: inject('session'),

  actions: {
    authenticate: function() {
      const credentials = this.getProperties('username', 'password');
      const authenticator = 'authenticator:jwt';

      this.get('session').authenticate(authenticator, credentials);
    }
  }
});

<!-- templates/u/login.hbs -->
<form >
  <label for="username">Login</label>
  
  <label for="password">Password</label>
  
  <button type="submit">Login</button>
</form>

I can make a successful request using Postman to Django, and I get the JWT back.

Submitting the login form with correct credentials gives a console error: Uncaught (in promise) Error: Token is empty. Please check your backend response.