Incoherence when using getattr in a field ObjPathSourceBinder

[idgserpro]

When I have a field ObjPathSourceBinder:

from plone.formwidget.contenttree import ObjPathSourceBinder
...

class ICarrossel(Interface):

    product = RelationChoice(
        title=_(u'Product'),
        source=ObjPathSourceBinder(object_provides=IProduct.__identifier__),
        required=False,
    )

When I create a Carrossel object that has never had the produtct field populated:

>>>print getattr(carrossel, 'product')
None

When I fill the product field, I then delete the product:

>>>print getattr(carrossel, 'product')
<z3c.relationfield.relation.RelationValue object at 0xb1289bac>