Invalid metadata reference not validated

heat_template_version: 2013-05-23

resources:
  instance:
    type: OS::Nova::Server
    #metadata: {'random': {get_attr: [random, value]}}
    metadata: {get_attr: [random, value]}
    properties:
      image: cirros-0.3.2-x86_64-disk
      flavor: m1.small
      key_name: stack_key

  random:
    type: OS::Heat::RandomString

If the (invalid) uncommented line is used, it passes validation, but then some time later the resource fails to store silently and the resource creation fails in an obscure way:

2014-07-07 23:29:06.452 TRACE heat.engine.resource File "/opt/stack/python-novaclient/novaclient/v1_1/servers.py", line 871, in create
2014-07-07 23:29:06.452 TRACE heat.engine.resource **boot_kwargs)
2014-07-07 23:29:06.452 TRACE heat.engine.resource File "/opt/stack/python-novaclient/novaclient/v1_1/servers.py", line 534, in _boot
2014-07-07 23:29:06.452 TRACE heat.engine.resource return_raw=return_raw, **kwargs)
2014-07-07 23:29:06.452 TRACE heat.engine.resource File "/opt/stack/python-novaclient/novaclient/base.py", line 152, in _create
2014-07-07 23:29:06.452 TRACE heat.engine.resource _resp, body = self.api.client.post(url, body=body)
2014-07-07 23:29:06.452 TRACE heat.engine.resource File "/opt/stack/python-novaclient/novaclient/client.py", line 286, in post
2014-07-07 23:29:06.452 TRACE heat.engine.resource return self._cs_request(url, 'POST', **kwargs)
2014-07-07 23:29:06.452 TRACE heat.engine.resource File "/opt/stack/python-novaclient/novaclient/client.py", line 260, in _cs_request
2014-07-07 23:29:06.452 TRACE heat.engine.resource **kwargs)
2014-07-07 23:29:06.452 TRACE heat.engine.resource File "/opt/stack/python-novaclient/novaclient/client.py", line 242, in _time_request
2014-07-07 23:29:06.452 TRACE heat.engine.resource resp, body = self.request(url, method, **kwargs)
2014-07-07 23:29:06.452 TRACE heat.engine.resource File "/opt/stack/python-novaclient/novaclient/client.py", line 236, in request
2014-07-07 23:29:06.452 TRACE heat.engine.resource raise exceptions.from_response(resp, body, url, method)
2014-07-07 23:29:06.452 TRACE heat.engine.resource BadRequest: Server name is not a string or unicode (HTTP 400) (Request-ID: req-cd388f26-5fbb-4364-85b0-bec49f1588ba)
2014-07-07 23:29:06.452 TRACE heat.engine.resource