exception: errorClass expected application tag enumerated

[Mathadon]

I'm getting this traceback but only occasionally when using bacpypes to run a bacnet server:

Traceback (most recent call last):

  File "bacpypes/appservice.py", line 1543, in confirmation

    xpdu.decode(apdu)

  File "bacpypes/apdu.py", line 712, in decode

    Sequence.decode(self, self._tag_list)

  File "bacpypes/constructeddata.py", line 232, in decode

    raise InvalidParameterDatatype("%s expected application tag %s" % (element.name, Tag._app_tag_name[element.klass._app_tag]))

bacpypes.errors.InvalidParameterDatatype: errorClass expected application tag enumerated

The relevant code snippet in constructeddata.py is :

                    if tag.tagClass != Tag.applicationTagClass or tag.tagNumber != element.klass._app_tag:
                        if not element.optional:
                            raise InvalidParameterDatatype("%s expected application tag %s" % (element.name, Tag._app_tag_name[element.klass._app_tag]))
                        else:
                            setattr(self, element.name, None)
                            continue

Is there an obvious fix for avoiding this exception?