django import error on startup of httpd

after starting httpd, the graphite web application is not available and the following error appears in the /opt/graphite/storage/log/webapp/error.log

ImproperlyConfigured: Error importing module django.contrib.auth.middleware: "cannot import name signals"

checking the directory usr/lib/python2.7/site-packages/django/db/models/ the following files are available:
signals.py signals.pyc signals.pyo

All python modules were installed using pip, so I am at a loss as to where I've gone wrong.