python - Django: migration to NullBooleanField fails with IntegrityError "contains null values" -

i'm working in django 1.7 , trying migrate database field called is_dispensing existing booleanfield nullbooleanfield.

my migration file:

# -*- coding: utf-8 -*- __future__ import unicode_literals  django.db import models, migrations  class migration(migrations.migration):      dependencies = [         ('frontend', '0007_practice_is_dispensing'),     ]      operations = [         migrations.alterfield(             model_name='practice',             name='is_dispensing',             field=models.nullbooleanfield(),             preserve_default=true,         ),     ] 

running manage.py migrate fails error:

django.db.utils.integrityerror: column "is_dispensing" contains null values 

the field in models file:

is_dispensing = models.nullbooleanfield(blank=true) 

previously was:

is_dispensing = models.booleanfield(null=true, blank=true)  

and when added it, asked provide default value, set none.

i find message confusing - i'm trying migrate column type nullbooleanfield, why can't contain null values? that's whole point of column type, isn't it? :)

update: thing that's confusing: if go postgres , @ table that's supposed have column, doesn't have is_dispensing column @ all.