Running gunicrn_start.sh with --preload: Error Empty module name - django

Question

I have a django project set up, with gunicorn. I have the following gunicorn_start:
#! /bin/bash
NAME="ostabs2"
DJANGODIR=/srv/osmium/ostabs2
SOCKFILE=/srv/osmium/run/gunicorn.sock
USER=tabuser
GROUP=webapps
NUM_WORKERS=3
DJANGO_SETTINGS_MODULE=ostabs2.settings
DANGO_WSGI_MODULE=ostabs2.wsgi
echo "Starting $NAME as 'whoami'"
cd $DJANGODIR
source /srv/osmium/venv/bin/activate
export DJANGO_SETTINGS_MODULE=$DJANGO_SETTINGS_MODULE
export PYTHONPATH=$DJANGODIR:$PYTHONPATH
exec gunicorn ${DJANGO_WSGI_MODULE}:application \
--preload \
--name $NAME \
--workers $NUM_WORKERS \
--user=$USER --group=$GROUP \
--log-level=debug \
--bind=unix:$SOCKFILE \
--log-file=ostabsgun.log
Running
gunicorn ostabs2.wsgi:application
on the command line works fine, but the gunicorn_start script gives:
Traceback (most recent call last):
File "/srv/osmium/venv/bin/gunicorn", line 11, in <module>
sys.exit(run())
File "/srv/osmium/venv/local/lib/python2.7/site-packages/gunicorn/app/wsgiapp.py", line 74, in run
WSGIApplication("%(prog)s [OPTIONS] [APP_MODULE]").run()
File "/srv/osmium/venv/local/lib/python2.7/site-packages/gunicorn/app/base.py", line 185, in run
super(Application, self).run()
File "/srv/osmium/venv/local/lib/python2.7/site-packages/gunicorn/app/base.py", line 71, in run
Arbiter(self).run()
File "/srv/osmium/venv/local/lib/python2.7/site-packages/gunicorn/arbiter.py", line 57, in __init__
self.setup(app)
File "/srv/osmium/venv/local/lib/python2.7/site-packages/gunicorn/arbiter.py", line 113, in setup
self.app.wsgi()
File "/srv/osmium/venv/local/lib/python2.7/site-packages/gunicorn/app/base.py", line 66, in wsgi
self.callable = self.load()
File "/srv/osmium/venv/local/lib/python2.7/site-packages/gunicorn/app/wsgiapp.py", line 65, in load
return self.load_wsgiapp()
File "/srv/osmium/venv/local/lib/python2.7/site-packages/gunicorn/app/wsgiapp.py", line 52, in load_wsgiapp
return util.import_app(self.app_uri)
File "/srv/osmium/venv/local/lib/python2.7/site-packages/gunicorn/util.py", line 356, in import_app
__import__(module)
ValueError: Empty module name
What would cause this error? Is there an error in the project, or a missing dependency? I have checked user permissions on all files and directories and tabuser has full read/write access. How can I track down the cause of this problem?
UPDATE: I have followed the advice from this question, the database definitions are correct (I can run syyncdb both locally and on the server). There are no empty strings in the installed apps section, and I have removed references throughout the app to libraries which are no longer used.
Is there anything else which can cause this error?