Can't find static files for the Django admin interface

Question

I can't find an answer to what seems like a simple question.

I have a Django 1.5 app on Heroku using staticfiles. The static files defined within my app can be found fine:

$ heroku run python manage.py findstatic bootstrap.min.css
Running `python manage.py findstatic bootstrap.min.css` attached to terminal... up, run.5557
Found 'bootstrap.min.css' here:
  /app/myapp/static/bootstrap.min.css

But the static files for the built-in admin interface can't be found, e.g.

$ heroku run python manage.py findstatic base.css  
Running `python manage.py findstatic base.css` attached to terminal... up, run.4546
No matching file found for 'base.css'.

The relevant bit of my settings is, I guess, this:

STATICFILES_FINDERS = (
    'django.contrib.staticfiles.finders.FileSystemFinder',
    'django.contrib.staticfiles.finders.AppDirectoriesFinder',
)

For clarity, everything else on the admin interface works, but not the static CSS files. If I visit the admin login page, the server logs have:

No such file or directory: 'staticfiles/admin/css/base.css'
No such file or directory: 'staticfiles/admin/css/login.css'