Error when using custom user model

I get the following error on the documentation root page:
ImproperlyConfigured: Field name usernameis not valid for modelCustomUser.

I imagine this has something to do with my custom user model, where I have disabled the username field by setting USERNAME_FIELD = 'email' in the model. The rest API works fine and it is only when setting url(r'^api/v1/', include('rest_framework_docs.urls')), that I get the problem at /api/v1/

The traceback indicates it is coming from the following lines in api_endpoint.py:

fields = [{
                "name": key,
                "type": str(field.__class__.__name__),
                "required": field.required
            } for key, field in serializer().get_fields().items()]

However, I can enter the python shell, get an instance of the serializer and run this code manually just fine. So I'm now at a loss as to what is causing this...

The full traceback is:

Traceback (most recent call last):
  File "/usr/local/lib/python2.7/site-packages/django/contrib/staticfiles/handlers.py", line 63, in __call__
    return self.application(environ, start_response)
  File "/usr/local/lib/python2.7/site-packages/django/core/handlers/wsgi.py", line 177, in __call__
    response = self.get_response(request)
  File "/usr/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 230, in get_response
    response = self.handle_uncaught_exception(request, resolver, sys.exc_info())
  File "/usr/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 289, in handle_uncaught_exception
    return debug.technical_500_response(request, *exc_info)
  File "/usr/local/lib/python2.7/site-packages/django_extensions/management/technical_response.py", line 6, in null_technical_500_response
    six.reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 149, in get_response
    response = self.process_exception_by_middleware(e, request)
  File "/usr/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 147, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/local/lib/python2.7/site-packages/django/utils/decorators.py", line 184, in inner
    return func(*args, **kwargs)
  File "/usr/local/lib/python2.7/site-packages/django/views/generic/base.py", line 68, in view
    return self.dispatch(request, *args, **kwargs)
  File "/usr/local/lib/python2.7/site-packages/django/views/generic/base.py", line 88, in dispatch
    return handler(request, *args, **kwargs)
  File "/usr/local/lib/python2.7/site-packages/django/views/generic/base.py", line 157, in get
    context = self.get_context_data(**kwargs)
  File "/usr/local/lib/python2.7/site-packages/rest_framework_docs/views.py", line 17, in get_context_data
    docs = ApiDocumentation()
  File "/usr/local/lib/python2.7/site-packages/rest_framework_docs/api_docs.py", line 21, in __init__
    self.get_all_view_names(root_urlconf.urlpatterns)
  File "/usr/local/lib/python2.7/site-packages/rest_framework_docs/api_docs.py", line 27, in get_all_view_names
    self.get_all_view_names(urlpatterns=pattern.url_patterns, parent_pattern=parent_pattern)
  File "/usr/local/lib/python2.7/site-packages/rest_framework_docs/api_docs.py", line 29, in get_all_view_names
    api_endpoint = ApiEndpoint(pattern, parent_pattern)
  File "/usr/local/lib/python2.7/site-packages/rest_framework_docs/api_endpoint.py", line 19, in __init__
    self.fields = self.__get_serializer_fields__()
  File "/usr/local/lib/python2.7/site-packages/rest_framework_docs/api_endpoint.py", line 54, in __get_serializer_fields__
    } for key, field in serializer().get_fields().items()]
  File "/usr/local/lib/python2.7/site-packages/rest_framework/serializers.py", line 1000, in get_fields
    field_name, info, model, depth
  File "/usr/local/lib/python2.7/site-packages/rest_framework/serializers.py", line 1135, in build_field
    return self.build_unknown_field(field_name, model_class)
  File "/usr/local/lib/python2.7/site-packages/rest_framework/serializers.py", line 1242, in build_unknown_field
    (field_name, model_class.__name__)
ImproperlyConfigured: Field name `username` is not valid for model `CustomUser`.

My serializer:

class CustomUserSerializer(serializers.ModelSerializer):
    client_profile = ClientProfileSerializer(required=False)
    instructor_profile = InstructorProfileSerializer(required=False)
    lookup_field = 'slug'

    class Meta:
        model = models.CustomUser
        fields = '__all__'

My model:

class OffPeaksUser(auth_models.AbstractBaseUser, auth_models.PermissionsMixin):
    '''Custom user model
    '''
    objects = UserManager()
    # Fields
    email = models.EmailField(max_length=255, unique=True)
    first_name = models.CharField(max_length=20)
    last_name = models.CharField(max_length=20)
    slug = models.SlugField(max_length=200, unique=True, null=True)
    profile_image = models.ImageField(blank=True, null=True)
    title = models.CharField(max_length=10, blank=True, null=True)
    biography = models.TextField(blank=True, null=True)
    newsletter = models.BooleanField(default=False)

    USERNAME_FIELD = 'email'
    REQUIRED_FIELDS = ['first_name', 'last_name']

Start in api_endpoint.py at get_serializer_fields, then trace how ApiDocumentation in api_docs.py discovers endpoints. Reproduce the /api/v1/ failure with a custom user model using USERNAME_FIELD = 'email' and compare it with serializer field inspection in the shell. Done means the documentation root loads without ImproperlyConfigured.