Problem when using alembic with sqlalchemy_utils

In my sqlalchemy model, I am using choicetype sqlalchemy_utils:

id = db.Column(db.Integer, primary_key=True)
code = db.Column(db.Integer, nullable=True)
level = db.Column(mytypes.types.ChoiceType(LEVEL))

      

        
        
        
      

    

I did everything as described here http://alembic.readthedocs.org/en/latest/autogenerate.html#autogen-module-prefix . In my model, I imported the choicetype from my mytypes.types module:

from sqlalchemy_utils.types.choice import ChoiceType

      

        
        
        
      

    

added context to alembic / env.py

context.configure(
    connection=connection,
    target_metadata=target_metadata,
    user_module_prefix="mytypes.types."
    # ...
)

      

        
        
        
      

    

and in script.py.mako

import mytypes.types

      

        
        
        
      

    

... The problem is when I do a revision of my model I get something like this

from alembic import op
import sqlalchemy as sa
import mytypes.types

def upgrade():
### commands auto generated by Alembic - please adjust! ###
op.add_column('logging', sa.Column('level', mytypes.types.ChoiceType(length=255), nullable=True))
### end Alembic commands ###

      

        
        
        
      

    

Why didn't alembic pass the "LEVEL" argument to the choicetype and why did it pass length = 255 instead?