Error on serializing zope.schema.List field

Hi,

I am using the Plone 5.2.2 Setup with
plone.restapi = 6.13.7 and
plone.app.vocabularies = 4.2.0

When requesting a certain object with fullobject=1 parameter via restapi, the plone.restapi plugin throws an exception with the following stack trace:

  Module ZPublisher.WSGIPublisher, line 162, in transaction_pubevents
  Module ZPublisher.WSGIPublisher, line 359, in publish_module
  Module ZPublisher.WSGIPublisher, line 262, in publish
  Module ZPublisher.mapply, line 85, in mapply
  Module ZPublisher.WSGIPublisher, line 63, in call_object
  Module plone.rest.service, line 22, in __call__
  Module plone.restapi.services, line 21, in render
  Module plone.restapi.services.content.get, line 18, in reply
  Module plone.restapi.serializer.dxcontent, line 161, in __call__
  Module plone.restapi.serializer.dxcontent, line 94, in __call__
  Module plone.restapi.serializer.dxfields, line 80, in __call__
AttributeError: 'CatalogVocabulary' object has no attribute 'getTerm'

The schema definition is as follows:

from zope.schema import List
from z3c.relationfield.schema import RelationChoice
from plone.app.vocabularies.catalog import CatalogSource

class ITopic(model.Schema):
    topics = List(
        title=_(u'topics.title', u'Topics'),
        required=True,
        min_length=1,
        value_type=RelationChoice(source=CatalogSource(portal_type=['Topics'])),
    )

Can anyone help me please? Many thanks in advance.

Start at the plone.restapi.serializer.dxfields entry point shown in the traceback and reproduce the request with fullobject=1 against the schema's List field using CatalogSource. Trace how the CatalogVocabulary is handled during serialization; done means the request completes without the getTerm AttributeError and the list field is serialized.