MTI traceback with explicit pk

gistfile1.pytb

>>> from bar.models import Bar
>>> b=Bar()
>>> b.pk='abcdef'
>>> b.save()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/home/aron/.virtualenvs/pp/lib/python2.7/site-packages/django/db/models/base.py", line 460, in save
    self.save_base(using=using, force_insert=force_insert, force_update=force_update)
  File "/home/aron/.virtualenvs/pp/lib/python2.7/site-packages/django/db/models/base.py", line 553, in save_base
    result = manager._insert(values, return_id=update_pk, using=using)
  File "/home/aron/.virtualenvs/pp/lib/python2.7/site-packages/django/db/models/manager.py", line 195, in _insert
    return insert_query(self.model, values, **kwargs)
  File "/home/aron/.virtualenvs/pp/lib/python2.7/site-packages/django/db/models/query.py", line 1436, in insert_query
    return query.get_compiler(using=using).execute_sql(return_id)
  File "/home/aron/.virtualenvs/pp/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 791, in execute_sql
    cursor = super(SQLInsertCompiler, self).execute_sql(None)
  File "/home/aron/.virtualenvs/pp/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 735, in execute_sql
    cursor.execute(sql, params)
  File "/home/aron/.virtualenvs/pp/lib/python2.7/site-packages/django/db/backends/util.py", line 34, in execute
    return self.cursor.execute(sql, params)
  File "/home/aron/.virtualenvs/pp/lib/python2.7/site-packages/django/db/backends/sqlite3/base.py", line 234, in execute
    return Database.Cursor.execute(self, query, params)
IntegrityError: bar_bar.foo_ptr_id may not be NULL

models.py

from django.db import models

class Foo(models.Model):
    id = models.CharField(max_length=6, primary_key=True)

class Bar(Foo):
    data = models.TextField()