aggregator raise an TypeError exception

Hi,

When the aggregator is used for aggregate swift outgoing or incoming bytes and publicly readable swift containers are accessed (without authentication) the following traceback occurs:

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/ceilometer/pipeline.py", line 267, in _transform_sample
    sample = transformer.handle_sample(ctxt, sample)
  File "/usr/lib/python2.7/dist-packages/ceilometer/transformer/conversions.py", line 227, in handle_sample
    key = self._get_unique_key(sample)
  File "/usr/lib/python2.7/dist-packages/ceilometer/transformer/conversions.py", line 217, in _get_unique_key
    for field in self.key_attributes])
TypeError: sequence item 0: expected string, NoneType found

My pipeline.yaml
---
sources:
    - name: meter_as_is
      interval: 1800
      meters:
          - "network.incoming.bytes"
          - "network.outgoing.bytes"
          - "storage.objects.size"
          - "volume.size"
      sinks:
          - meter_sink
    - name: swift_traffic_outgoing_source
      interval: 10
      meters:
          - "storage.objects.outgoing.bytes"
      sinks:
          - swift_traffic_outgoing_sink
sinks:
    - name: meter_sink
      transformers:
      publishers:
          - rpc://
    - name: swift_traffic_outgoing_sink
      transformers:
          - name: "aggregator"
            parameters:
                size: 1000
                retention_time: 1800
                resource_metadata: "drop"
                tenant_id: 'first'
                target:
                    name: "storage.objects.outgoing.bytes.aggregated"
      publishers:
          - rpc://