[web2py] Field update attribute broken?

[baloan]

Hi,

I have defined a table:

username = auth.user and auth.user.username

db.define_table('email',
                Field('email', 'string', length=40, notnull=False,
unique=True, represent=lambda s: s),
                Field('active', 'boolean', default=True),
                Field('expires', 'date', default=None,
represent=lambda s: s or 'Never'),
                Field('create_user', 'string', default=username,
writable=False),
                Field('create_date', 'datetime', default=request.now,
writable=False),
                Field('change_user', 'string', update=username,
writable=False),
                Field('change_date', 'datetime', update=request.now,
writable=False),
                )

and get the following form on crud.create():

Email:          []
Active:                 [x]
Expires:                []
Create User:    baloan
Create Date:    2010-11-12 21:53:58
Change User:    baloan
Change Date:    2010-11-12 21:53:58
[Submit]

Though not specified by using 'default', change user/date is filled
with username and request.now. It doesn't change if I use
default=None.

Is there any way to leave Change User/Date empty with crud.create?
Maybe I have a database table synchronisation problem? Where is
default/update information kept?

Regards, Andreas