Why are "Models aren't loaded yet"?

I am trying to install django-registration-redux with a customUser.

I have included this in my settings.py:

AUTH_USER_MODEL = 'app.customUser'

Registration Form is in a directory ../registration/forms.py:

from __future__ import unicode_literals


from django import forms
from django.utils.translation import ugettext_lazy as _
from django.contrib.auth.forms import UserCreationForm

from .users import UserModel, UsernameField

User = UserModel()


class RegistrationForm(UserCreationForm):

    required_css_class = 'required'
    email = forms.EmailField(label=_("E-mail"))

    class Meta:
        model = customUser
        fields = ('email')

Additionally, models.py has the following:

from __future__ import unicode_literals
from django.db import models

# Create your models here.

import datetime
import hashlib
import random
import re

from django.conf import settings
from django.core.mail import EmailMultiAlternatives
from django.db import models
from django.template import RequestContext, TemplateDoesNotExist
from django.template.loader import render_to_string
from django.utils.translation import ugettext_lazy as _
from django.utils.encoding import python_2_unicode_compatible
from django.utils.timezone import now as datetime_now
from django.utils import six

from .users import UserModel, UserModelString


from django.contrib.auth.models import BaseUserManager, AbstractBaseUser
from django.contrib.auth.admin import UserAdmin


class customUserManager(BaseUserManager):
    def create_user(self, email, password=None):
        """
        Creates and saves a User with the given email, date of
    birth and password.
        """
        if not email:
            raise ValueError('Users must have an email address')

        user = self.model(
            email=self.normalize_email(email),
        )

        user.set_password(password)
        user.save(using=self._db)
        return user

    def create_superuser(self, email, password):
        """
        Creates and saves a superuser with the given email, date of
    birth and password.
        """
        user = self.create_user(email,
            password=password,
        )
        user.is_active = True
        user.is_admin = True
        user.save(using=self._db)
        return user

class customUser(AbstractBaseUser):
    email = models.EmailField(verbose_name='email address', max_length=255, unique=True)
    first_name = models.CharField(max_length=50, null=True)
    middle_name = models.CharField(max_length=50, null=True)
    last_name = models.CharField(max_length=50, null=True)
    is_active = models.BooleanField(default=False)
    is_admin = models.BooleanField(default=False)

    objects = customUserManager()

    USERNAME_FIELD = 'email'


class customUserAdmin(UserAdmin):
    # The forms to add and change user instances
    #form = UserChangeForm
    #add_form = RegisterationForm

class RegistrationManager(models.Manager):
    """
    """

Here is the Traceback:

Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/django/core/management/__init__.py", line 338, in execute_from_command_line
utility.execute()
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/django/core/management/__init__.py", line 312, in execute
django.setup()
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/django/__init__.py", line 18, in setup
apps.populate(settings.INSTALLED_APPS)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/django/apps/registry.py", line 108, in populate
app_config.import_models(all_models)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/django/apps/config.py", line 198, in import_models
self.models_module = import_module(models_module_name)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/importlib/__init__.py", line 37, in import_module
__import__(name)
  File "/Users/Desktop/app/sites/models.py", line 21, in <module>
from .users import UserModel, UserModelString
  File "/Users/Desktop/app/sites/users.py", line 4, in <module>
UserModel = get_user_model()
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/django/contrib/auth/__init__.py", line 150, in get_user_model
return django_apps.get_model(settings.AUTH_USER_MODEL)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/django/apps/registry.py", line 199, in get_model
self.check_models_ready()
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/django/apps/registry.py", line 131, in check_models_ready
raise AppRegistryNotReady("Models aren't loaded yet.")
django.core.exceptions.AppRegistryNotReady: Models aren't loaded yet.

If I remove the () in get_user_model in my users.py, I get this error:

users.py

from django.conf import settings
from django.contrib.auth import get_user_model

UserModel = get_user_model

def UserModelString():
    try:
        return settings.AUTH_USER_MODEL
    except AttributeError:
        return 'auth.User'


def UsernameField():
    return getattr(UserModel(), 'USERNAME_FIELD', 'email')

Error:

File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/django/contrib/auth/__init__.py", line 155, in get_user_model
"AUTH_USER_MODEL refers to model '%s' that has not been installed" % settings.AUTH_USER_MODEL
django.core.exceptions.ImproperlyConfigured: AUTH_USER_MODEL refers to model 'app.customUser' that has not been installed