Django custom user model in admin, relation “auth_user” does not exist

I have a custom user model as below:

class User(AbstractUser):
    subscribe_newsletters = models.BooleanField(default=True)
    old_id = models.IntegerField(null=True, blank=True)
    old_source = models.CharField(max_length=25, null=True, blank=True)

And using the builtin UserAdmin

admin.site.register(User, UserAdmin)

While editing the user record works fine, but when I add a user, I get the following error

Exception Value: 
relation "auth_user" does not exist
LINE 1: ...user"."is_active", "auth_user"."date_joined" FROM "auth_user...

After some digging around and found this https://docs.djangoproject.com/en/1.5/topics/auth/customizing/#custom-users-and-the-built-in-auth-forms

The culprit is a function clean_uesrname inside UserCreationForm inside django.contrib.auth.forms.py
A few tickets have been created, but apparently the maintainers don’t think it’s a defect

https://code.djangoproject.com/ticket/20188
https://code.djangoproject.com/ticket/20086

def clean_username(self):
# Since User.username is unique, this check is redundant,
# but it sets a nicer error message than the ORM. See #13147.
username = self.cleaned_data[“username”]
try:
User._default_manager.get(username=username)
except User.DoesNotExist:
return username
raise forms.ValidationError(self.error_messages[‘duplicate_username’])
The User in this file is directly referencing to the builtin user model.

To fix it, I created my custom forms

from models import User #you can use get_user_model
from django.contrib.auth.forms import UserCreationForm
from django.contrib.auth.admin import UserAdmin

class MyUserCreationForm(UserCreationForm):
    def clean_username(self):
        # Since User.username is unique, this check is redundant,
        # but it sets a nicer error message than the ORM. See #13147.
        username = self.cleaned_data["username"]
        try:
            User._default_manager.get(username=username)
        except User.DoesNotExist:
            return username
        raise forms.ValidationError(self.error_messages['duplicate_username'])

    class Meta(UserCreationForm.Meta):
        model = User

class MyUserAdmin(UserAdmin):  
    add_form = MyUserCreationForm   

admin.site.register(User,MyUserAdmin)