Getting 'DatabaseOperations' object has no attribute 'geo_db_type' error when doing a syncdb

I'm attempting to run heroku run python manage.py syncdb on my GeoDjango app on Heroku, but I get the following error:

AttributeError: 'DatabaseOperations' object has no attribute 'geo_db_type'

All of my research has yielded the same solution: make sure to use django.contrib.gis.db.backends.postgis as the database engine. Funny thing is that I'm already doing this (and I also have django.contrib.gis in INSTALLED_APPS):

settings.py

DATABASES = {
  'default': {
    'ENGINE': 'django.contrib.gis.db.backends.postgis',
    'NAME': '...',
    'HOST': '...',
    'PORT': ...,
    'USER': '...',
    'PASSWORD': '...'
  }
}

INSTALLED_APPS = (
    ...,
    'django.contrib.gis',
)

Is there something else I am missing? Any help is greatly appreciated, below is the full error trace for reference:

Running `python manage.py syncdb` attached to terminal... up, run.1
Creating tables ...
Creating table auth_permission
Creating table auth_group_permissions
Creating table auth_group
Creating table auth_user_user_permissions
Creating table auth_user_groups
Creating table auth_user
Creating table django_content_type
Creating table django_session
Creating table django_site
Creating table django_admin_log
Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/app/lib/python2.7/site-packages/django/core/management/__init__.py", line 443, in execute_from_command_line
    utility.execute()
  File "/app/lib/python2.7/site-packages/django/core/ma