Error reported even network is specified

[JunJie Nan]

How to reproduce:
1. test_server_networks.yaml
heat_template_version: 2013-05-23

parameters:
  flavor:
    type: string
    default: m1.small
  image:
    type: string
    default: ubuntu-amd64
  key:
    type: string
    default: heat_key

resources:
  network:
    type: test_server_networks_backnet.yaml
  server:
    type: OS::Nova::Server
    properties:
      flavor: {get_param: flavor}
      image: {get_param: image}
      key_name: {get_param: key}
      networks:
        - network: {get_attr: [network, back_net]}
outputs:
  result:
    value: passed
    description: result

2. filename: test_server_networks.yaml
heat_template_version: 2013-05-23

parameters:
  back_cidr:
    type: string
    description: backend subnet cidr
    default: 172.16.200.0/24

resources:
  backend:
    type: OS::Neutron::Net
  backend_subnet:
    type: OS::Neutron::Subnet
    properties:
      network_id: {get_resource: backend}
      cidr: {get_param: back_cidr}
      dns_nameservers: [8.8.8.8]

outputs:
  back_net:
    description: backend internal network
    value: {get_resource: backend}
  back_subnet:
    description: backend internal subnet
    value: {get_resource: backend_subnet}

3. heat stack-create -f test_server_networks.yaml test_server_networks

Expected result:
should work
Actual result:
ERROR: One of the properties "network", "port", "uuid" should be set for the specified network of server "server".
You can see the network is specified actually.