HStoreSerializer breaks nullbooleanfield

[mc-multi-it-dk]

File "/home/max/git/upgrades/env/lib/python2.7/site-packages/rest_framework/fields.py", line 643, in init
assert 'allow_null' not in kwargs, 'allow_null is not a valid option.'
AssertionError: allow_null is not a valid option.

solution:
changed line 39 in serializers.py
from:
if model_field.null:
to:
if model_field.null and not isinstance(model_field, models.NullBooleanField):