"Object is of wrong type" with NamedBlogImage field

Plone 4.3, Dexterity 2.

I have an image field

    image = NamedBlobImage(
       title=_(u'label_special_image'),
       description=_(u'help_special_image'),
       required=False,
    )

which is updated during a migration using

                if new_obj.portal_type == 'Image':
                    setattr(new_obj, k, namedfile.NamedBlobImage(v,
filename=filename))


The image is stored correctly and shown correctly in our own custom
view. When we @@edit the content-type instance and save then we
receive an error "Object is of wrong type" (see screenshot).
The preview of the image is also being displayed correctly.

Why is this happening?

Andreas

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