heroku-buildpack-python: cannot import name 'GDALRaster'

Hi. I’m having trouble configuring a heroku-16 instance to support GeoDjango. I’ve followed the documentation on the Heroku site closely, but always encounter the error:

django.core.exceptions.ImproperlyConfigured: 'django.contrib.gis.db.backends.postgis' isn't an available database backend.
Try using 'django.db.backends.XXX', where XXX is one of:
    'mysql', 'oracle', 'postgresql', 'sqlite3'
Error was: cannot import name 'GDALRaster'

Could this be related to any of the recent issues/changes related to this repo having to do with GIS dependencies like GDAL and GEOS. Thanks for any ideas.