SQLAlchemy bulk insertion with None

bulk_none.py

from sqlalchemy import Column, String, Integer
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy.sql.expression import null

Base = declarative_base()

class Doc(Base):
    __tablename__ = 'doc'
    id = Column(Integer, primary_key=True)
    col_a = Column(String)
    col_b = Column(String)

engine = create_engine('sqlite:///', echo=True)
DBSession = sessionmaker()
DBSession.configure(bind=engine)
Base.metadata.drop_all(engine, checkfirst=True)
Base.metadata.create_all(engine, checkfirst=True)

session = DBSession()
session.bulk_insert_mappings(Doc, [
    dict(id=1, col_a='A', col_b='B'),
    dict(id=2, col_a='A', col_b=None),
    dict(id=3, col_a='A', col_b='B')
])
session.commit()