addition of a trunk subport in a dict fails

OPNFV functest neutron-trunk's test_remove_subport_passing_dict test case fails. What this test case tries is to add the subport as a dict:

@decorators.attr(type='negative')
    @decorators.idempotent_id('17ca7dd7-96a8-445a-941e-53c0c86c2fe2')
    def test_remove_subport_passing_dict(self):
        network = self.create_network()
        parent_port = self.create_port(network)
        subport_data = {'port_id': parent_port['id'],
                        'segmentation_type': 'vlan',
                        'segmentation_id': 2}
        trunk = self._create_trunk_with_network_and_parent([subport_data])
        self.assertRaises(lib_exc.BadRequest, self.client.remove_subports,
                          trunk['trunk']['id'], subport_data)

Here's the error from the trunk plugin:

2019-10-10 09:46:29.008 35019 DEBUG networking_vpp.services.trunk.trunk_vpp [req-6ac8e4ba-6d6e-4030-a9c4-b995720a81ec d05e959fd1cc4d08be25bec57541adbd 09ee7fc64a5a41e0b16d0a3aa15c0269 - default default] Removing subports {u'sub_ports': {u'segmentation_type': u'vlan', u'port_id': u'761f8090-8215-4730-bcee-3d74bea78215', u'segmentation_id': 2}} from trunk dde9305d-01ba-4201-b376-a7b49ff6ff60 remove_subports /usr/local/lib/python2.7/dist-packages/networking_vpp/services/trunk/trunk_vpp.py:471
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource [req-6ac8e4ba-6d6e-4030-a9c4-b995720a81ec d05e959fd1cc4d08be25bec57541adbd 09ee7fc64a5a41e0b16d0a3aa15c0269 - default default] remove_subports failed: No details.: TypeError: string indices must be integers
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource Traceback (most recent call last):
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/dist-packages/neutron/api/v2/resource.py", line 98, in resource
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource result = method(request=request, **args)
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/dist-packages/neutron_lib/db/api.py", line 140, in wrapped
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource setattr(e, '_RETRY_EXCEEDED', True)
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/dist-packages/oslo_utils/excutils.py", line 220, in __exit__
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource self.force_reraise()
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/dist-packages/oslo_utils/excutils.py", line 196, in force_reraise
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource six.reraise(self.type_, self.value, self.tb)
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/dist-packages/neutron_lib/db/api.py", line 136, in wrapped
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource return f(*args, **kwargs)
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/dist-packages/oslo_db/api.py", line 154, in wrapper
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource ectxt.value = e.inner_exc
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/dist-packages/oslo_utils/excutils.py", line 220, in __exit__
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource self.force_reraise()
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/dist-packages/oslo_utils/excutils.py", line 196, in force_reraise
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource six.reraise(self.type_, self.value, self.tb)
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/dist-packages/oslo_db/api.py", line 142, in wrapper
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource return f(*args, **kwargs)
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/dist-packages/neutron_lib/db/api.py", line 183, in wrapped
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource LOG.debug("Retry wrapper got retriable exception: %s", e)
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/dist-packages/oslo_utils/excutils.py", line 220, in __exit__
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource self.force_reraise()
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/dist-packages/oslo_utils/excutils.py", line 196, in force_reraise
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource six.reraise(self.type_, self.value, self.tb)
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/dist-packages/neutron_lib/db/api.py", line 179, in wrapped
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource return f(*dup_args, **dup_kwargs)
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/dist-packages/neutron/api/v2/base.py", line 253, in _handle_action
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource ret_value = getattr(self._plugin, name)(*arg_list, **kwargs)
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/dist-packages/neutron/db/db_base_plugin_common.py", line 49, in inner
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource result = f(*args, **kwargs)
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource File "/usr/local/lib/python2.7/dist-packages/networking_vpp/services/trunk/trunk_vpp.py", line 477, in remove_subports
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource subports_to_remove = [pid['port_id'] for pid in subports['sub_ports']]
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource TypeError: string indices must be integers
2019-10-10 09:46:29.018 35019 ERROR neutron.api.v2.resource

Here's the debug log which shows what is being sent to neutron from the test case:

2019-10-10 09:48:26.651 391 INFO tempest.lib.common.rest_client [req-435f5564-5553-4d2b-8b73-0d50ea69d6d6 ] Request (TrunkTestJSON:test_remove_subport_passing_dict): 201 POST http://172.16.10.36:9696/v2.0/trunks 0.345s
2019-10-10 09:48:26.651 391 DEBUG tempest.lib.common.rest_client [req-435f5564-5553-4d2b-8b73-0d50ea69d6d6 ] Request - Headers: {'Content-Type': 'application/json', 'Accept': 'application/json', 'X-Auth-Token': '<omitted>'}
        Body: {"trunk": {"sub_ports": [{"segmentation_type": "vlan", "port_id": "761f8090-8215-4730-bcee-3d74bea78215", "segmentation_id": 2}], "port_id": "004ac8ef-3047-4296-8851-e83499cb588a"}}
    Response - Headers: {'status': '201', u'content-length': '509', 'content-location': 'http://172.16.10.36:9696/v2.0/trunks', u'date': 'Thu, 10 Oct 2019 09:46:28 GMT', u'content-type': 'application/json', u'connection': 'close', u'x-openstack-request-id': 'req-435f5564-5553-4d2b-8b73
        Body: {"trunk": {"status": "DOWN", "sub_ports": [{"segmentation_type": "vlan", "port_id": "761f8090-8215-4730-bcee-3d74bea78215", "segmentation_id": 2}], "name": "", "admin_state_up": true, "tenant_id": "09ee7fc64a5a41e0b16d0a3aa15c0269", "created_at": "2019-10-10T09:46:28Z", "
2019-10-10 09:48:26.682 391 INFO tempest.lib.common.rest_client [req-6ac8e4ba-6d6e-4030-a9c4-b995720a81ec ] Request (TrunkTestJSON:test_remove_subport_passing_dict): 500 PUT http://172.16.10.36:9696/v2.0/trunks/dde9305d-01ba-4201-b376-a7b49ff6ff60/remove_subports 0.029s
2019-10-10 09:48:26.682 391 DEBUG tempest.lib.common.rest_client [req-6ac8e4ba-6d6e-4030-a9c4-b995720a81ec ] Request - Headers: {'Content-Type': 'application/json', 'Accept': 'application/json', 'X-Auth-Token': '<omitted>'}
        Body: {"sub_ports": {"segmentation_type": "vlan", "port_id": "761f8090-8215-4730-bcee-3d74bea78215", "segmentation_id": 2}}
    Response - Headers: {'status': '500', u'content-length': '150', 'content-location': 'http://172.16.10.36:9696/v2.0/trunks/dde9305d-01ba-4201-b376-a7b49ff6ff60/remove_subports', u'date': 'Thu, 10 Oct 2019 09:46:29 GMT', u'content-type': 'application/json', u'connection': 'close', u'
        Body: {"NeutronError": {"message": "Request Failed: internal server error while processing your request.", "type": "HTTPInternalServerError", "detail": ""}} _log_request_full /root/.rally/verification/verifier-4d110db8-b995-41ae-b4f2-1c23118eed79/repo/tempest/lib/common/rest_cl