Spurious failures in django.contrib.messages.tests when running manage.py test

Question

I've recently added authentication (via django.contrib.auth of course) to my application, along with appropriate "signin"/"signup" links to my base.html.

The problem comes when I run manage.py tests, and I get 4 failures, all from django.contrib.messages.tests:

ERROR: test_middleware_disabled_anon_user (django.contrib.messages.tests.cookie.CookieTest)
ERROR: test_middleware_disabled_anon_user (django.contrib.messages.tests.fallback.FallbackTest)
ERROR: test_middleware_disabled_anon_user (django.contrib.messages.tests.user_messages.LegacyFallbackTest)
ERROR: test_middleware_disabled_anon_user (django.contrib.messages.tests.session.SessionTest)

All with the same failure:

TemplateSyntaxError: Caught NoReverseMatch while rendering: Reverse for 'django.contrib.auth.views.login' with arguments '()' and keyword arguments '{}' not found.

In manage.py shell this works:

>>> from django.core.urlresolvers import reverse
>>> reverse('django.contrib.auth.views.login')
'/signin/'

However this doesn't:

>>> reverse('django.contrib.auth.views.login', (), {})
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/Users/dave/Dropbox/Projects/statbooks.co.uk/lib/python2.6/site-packages/django/core/urlresolvers.py", line 350, in reverse
    *args, **kwargs)))
  File "/Users/dave/Dropbox/Projects/statbooks.co.uk/lib/python2.6/site-packages/django/core/urlresolvers.py", line 296, in reverse
    "arguments '%s' not found." % (lookup_view_s, args, kwargs))
NoReverseMatch: Reverse for 'django.contrib.auth.views.login' with arguments '()' and keyword arguments '{}' not found.

Commenting out the {% url %} tags from my base.html make the tests pass.

What's causing this?