In my project I am using PostgreSQL/PostGIS as the database and Django with django.contrib.gis configured. The existing table pois contains geo         
        
The database configuration in settings.py was incorrect:
DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.postgresql_psycopg2',
        'NAME': 'geodjango',
        'USER': 'geo',
        'PASSWORD': 'secret',
        'HOST': 'localhost',
        'PORT': '5432',
    }
}
Correct:
DATABASES = {
    'default': {
        'ENGINE': 'django.contrib.gis.db.backends.postgis', # Here
        'NAME': 'geodjango',
        'USER': 'geo',
        'PASSWORD': 'secret',
        'HOST': 'localhost',
        'PORT': '5432',
    }
}
I have overseen this in the documentation somehow. Thanks to apollo13 from the #django irc channel for the advice into the right direction.