Django-rest-framework: AttributeError: 'OrderedDict'-Objekt hat kein Attribut 'pk'

Ansichten:

@detail_route(methods=['post'])
def quote(self, request, pk):
    task = self.get_object()

    serializer = serializers.TaskCreateQuoteSerializer(data=request.DATA, task=task)

    if not serializer.is_valid():
        return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)

    # this causes a problem
    serializer.data['servicetasks']:

Serialisierer:

class TaskCreateQuoteSerializer(serializers.Serializer):
    servicetasks = serializers.PrimaryKeyRelatedField(many=True, required=True, allow_null=False,
        queryset=ServiceTask.objects.all())

    def __init__(self, task, *args, **kwargs):
        super(TaskCreateQuoteSerializer, self).__init__(*args, **kwargs)
        # print task.servicetask_set.all()
        # self.fields['servicetasks'].queryset = task.servicetask_set.all()

Wenn ich versuche, den Serializer zu drucken:

TaskCreateQuoteSerializer(data={u'servicetasks': [3115, 3114]}, task=<Task: E13/688 COMPLETE>):
    servicetasks = PrimaryKeyRelatedField(allow_null=False, many=True, queryset=[<ServiceTask: Ser...]

Wenn ich also versuche, auf serializer.data zuzugreifen, erhalte ich eine Ausnahme:

AttributeError: 'OrderedDict'-Objekt hat kein Attribut 'pk'

Traceback:
File "/usr/local/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
  111.                     response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "/usr/local/lib/python2.7/site-packages/django/views/decorators/csrf.py" in wrapped_view
  57.         return view_func(*args, **kwargs)
File "/usr/local/lib/python2.7/site-packages/rest_framework/viewsets.py" in view
  85.             return self.dispatch(request, *args, **kwargs)
File "/usr/local/lib/python2.7/site-packages/rest_framework/views.py" in dispatch
  452.             response = self.handle_exception(exc)
File "/usr/local/lib/python2.7/site-packages/rest_framework/views.py" in dispatch
  449.             response = handler(request, *args, **kwargs)
File "/usr/src/app/abas/apps/api/views.py" in quote
  270.         if not serializer.data['servicetasks']:
File "/usr/local/lib/python2.7/site-packages/rest_framework/serializers.py" in data
  466.         ret = super(Serializer, self).data
File "/usr/local/lib/python2.7/site-packages/rest_framework/serializers.py" in data
  215.                 self._data = self.to_representation(self.validated_data)
File "/usr/local/lib/python2.7/site-packages/rest_framework/serializers.py" in to_representation
  426.                 attribute = field.get_attribute(instance)
File "/usr/local/lib/python2.7/site-packages/rest_framework/relations.py" in get_attribute
  364.         if not instance.pk: