Cannot specify default value in field constructor

[earthfront]

What I want:

class MyModel(models.Model):
    duration = RelativeDeltaField(null=False, default='P1Y')

What error occurs:
"manage.py makemigrations" works, but during "manage.py migrate" I see:

...SNIP...
in convert_relativedelta_to_duration
    days=delta.days, seconds=delta.seconds,
AttributeError: 'str' object has no attribute 'days'

If I set the 'default' parameter to a dateutil.relativedelta.relativedelta type, an error occurs in 'manage.py makemigrations'

Am I doing this wrong? Documentation suggests that the only provided way to set the field is via the model constructor.