Should never get here. Please report a bug, including your model and model manager setup.

[Mikuláš Poul]

The unthinkable happened!

Traceback (most recent call last):
  File "manage.py", line 14, in <module>
    execute_from_command_line(sys.argv)
  File "{...}/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 350, in execute_from_command_line
    utility.execute()
  File "{...}/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 324, in execute
    django.setup()
  File "{...}/local/lib/python2.7/site-packages/django/__init__.py", line 18, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "{...}/local/lib/python2.7/site-packages/django/apps/registry.py", line 108, in populate
    app_config.import_models(all_models)
  File "{...}/local/lib/python2.7/site-packages/django/apps/config.py", line 202, in import_models
    self.models_module = import_module(models_module_name)
  File "/usr/lib/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
  File "{...}/budgetter-app/apps/expense/models.py", line 29, in <module>
    class Expense(models.Model):
  File "{...}/local/lib/python2.7/site-packages/django/db/models/base.py", line 283, in __new__
    new_class._prepare()
  File "{...}/local/lib/python2.7/site-packages/django/db/models/base.py", line 332, in _prepare
    ensure_default_manager(cls)
  File "{...}/local/lib/python2.7/site-packages/django/db/models/manager.py", line 45, in ensure_default_manager
    "Should never get here. Please report a bug, including your "
AssertionError: Should never get here. Please report a bug, including your model and model manager setup.

My model and manager setup

from django.db import models
from django.db.models.manager import BaseManager
from django.db.models.query import QuerySet

class ExpenseQuerySet(QuerySet):
   pass

class ExpenseManager(BaseManager.from_queryset(ExpenseQuerySet)):
    pass

class Expense(models.Model):
    objects = ExpenseManager()

[Mikuláš Poul]

Adding use_for_related_fields = True to ExpenseManager fixed the issue. But still...

[Tim Graham]

There's no crash in Django 1.10 as of ed0ff913c648b16c4471fc9a9441d1ee48cb5420, however, I'm not certain about the correct resolution of this ticket. For example, should we add an additional test for this case?

I edited the description to leave only the minimal code necessary to reproduce the issue.