fail to get nested output from OS::Heat::ResourceGroup resources

heat version: 9.0.0

template:

heat_template_version: 2017-02-24
description: test template
parameters:
  image_name:
    type: string
    constraints:
      - custom_constraint: glance.image
  flavor_name:
    type: string
    constraints:
      - custom_constraint: nova.flavor
resources:
  test_net:
    type: OS::Neutron::Net
  test_subnet:
    type: OS::Neutron::Subnet
    properties:
      network: { get_resource: test_net }
      ip_version: 4
      cidr: "192.168.1.0/24"
      gateway_ip: null
  test_server:
    type: OS::Heat::ResourceGroup
    properties:
      count: 1
      resource_def:
        type: OS::Nova::Server
        properties:
          user_data_format: RAW
          image: { get_param: image_name }
          flavor: { get_param: flavor_name }
          networks:
            - network: { get_resource: test_net }
outputs:
  test:
    value:
      get_attr:
        - test_server
        - addresses
        - get_resource: test_net
        - 0
        - addr

try to create stack and get:
ERROR: TypeError: : resources.test_server: : sequence item 1: expected string or Unicode, NoneType found

on heat 8.0.4 template works fine and outputs return array of addr

heat engine log:
2017-09-26 12:53:04.201 3732 DEBUG heat.engine.parameter_groups [req-a9a19cf8-69e6-44bf-9f1d-c1691da5412e - o3ltest - default default] Validating Parameter Groups: OS::project_id, image_name, OS::stack_id, flavor_name validate /usr/lib/python2.7/site-packages/heat/engine/parameter_groups.py:42
2017-09-26 12:53:04.203 3732 INFO heat.engine.resource [req-a9a19cf8-69e6-44bf-9f1d-c1691da5412e - o3ltest - default default] Validating Net "test_net"
2017-09-26 12:53:04.204 3732 INFO heat.engine.resource [req-a9a19cf8-69e6-44bf-9f1d-c1691da5412e - o3ltest - default default] Validating ResourceGroup "test_server"
2017-09-26 12:53:04.205 3732 INFO heat.engine.stack [req-a9a19cf8-69e6-44bf-9f1d-c1691da5412e - o3ltest - default default] Exception in stack validation
2017-09-26 12:53:04.205 3732 ERROR heat.engine.stack Traceback (most recent call last):
2017-09-26 12:53:04.205 3732 ERROR heat.engine.stack File "/usr/lib/python2.7/site-packages/heat/engine/stack.py", line 879, in validate
2017-09-26 12:53:04.205 3732 ERROR heat.engine.stack result = res.validate()
2017-09-26 12:53:04.205 3732 ERROR heat.engine.stack File "/usr/lib/python2.7/site-packages/heat/engine/resources/stack_resource.py", line 65, in validate
2017-09-26 12:53:04.205 3732 ERROR heat.engine.stack self.validate_nested_stack()
2017-09-26 12:53:04.205 3732 ERROR heat.engine.stack File "/usr/lib/python2.7/site-packages/heat/engine/resources/openstack/heat/resource_group.py", line 285, in validate_nested_stack
2017-09-26 12:53:04.205 3732 ERROR heat.engine.stack include_all=True)
2017-09-26 12:53:04.205 3732 ERROR heat.engine.stack File "/usr/lib/python2.7/site-packages/heat/engine/resources/openstack/heat/resource_group.py", line 567, in _assemble_nested
2017-09-26 12:53:04.205 3732 ERROR heat.engine.stack get_attr):
2017-09-26 12:53:04.205 3732 ERROR heat.engine.stack File "/usr/lib/python2.7/site-packages/heat/engine/resources/openstack/heat/resource_group.py", line 465, in _nested_output_defns
2017-09-26 12:53:04.205 3732 ERROR heat.engine.stack output_name = ', '.join(attr)
2017-09-26 12:53:04.205 3732 ERROR heat.engine.stack TypeError: sequence item 1: expected string or Unicode, NoneType found
2017-09-26 12:53:04.205 3732 ERROR heat.engine.stack