[web2py] extra_fields problem

[Cliff]

I get an exception from this code in db.py:

auth.settings.extra_fields['auth_user'].append(Field('created_by',
   default=session.auth.user.id), writable=False, readable=False)

Here is the exception:

Traceback (most recent call last):
  File "/media/Myfiles/w2p-1-98-2/web2py/gluon/restricted.py", line
192, in restricted
    exec ccode in environment
  File "/media/Myfiles/w2p-1-98-2/web2py/applications/Purchasing/
models/db.py", line 48, in <module>
    auth.settings.extra_fields['auth_user'].append(Field('created_by',
KeyError: 'auth_user'

The code is before the call to "define_tables()"

Am I doing something wrong or is this a Web2py problem?