Django 1.2 and invalid exclude definition in admin.py

[restless]

When I enter admin panel I get an error saying that "exclude refers to field created_on that is missing from the form."

The problem is caused by the following definition (admin.py):

class WorkflowAdmin(admin.ModelAdmin):
    """
    Workflow administration
    """
    (...)
    exclude = ['created_on', 'cloned_from']
    list_filter = ['status']

It is enough to remove created_on from the exclude (exclude = ['cloned_from']). created_on field is defined as auto_now_add which makes it automatically editable=False, so it is not included in the form generated for admin.