failed to describe address in ec2 api

unable to describe the address associated with an instance.

to reproduce:
# euca-describe-addresses
192.168.1.100
192.168.1.101
192.168.1.102

# euca-associate-address -i i-XXX 192.168.1.100
ADDRESS 192.168.1.100 i-XXX

# euca-describe-addresses
UnknownError: An unknown error has occurred. Please try your request again.

traceback:
2012-09-13 20:55:05 ERROR nova.api.ec2 [req-0895ee95-d972-4dc1-99a0-5e0eb3889261 1458120d63784bb49858437c275dc59e 7dd8607e7d294efc845ccf68f8d282b7] Unexpected error raised: 'instance_id'
2012-09-13 20:55:05 TRACE nova.api.ec2 Traceback (most recent call last):
2012-09-13 20:55:05 TRACE nova.api.ec2 File "/usr/lib/python2.7/dist-packages/nova/api/ec2/__init__.py", line 476, in __call__
2012-09-13 20:55:05 TRACE nova.api.ec2 result = api_request.invoke(context)
2012-09-13 20:55:05 TRACE nova.api.ec2 File "/usr/lib/python2.7/dist-packages/nova/api/ec2/apirequest.py", line 81, in invoke
2012-09-13 20:55:05 TRACE nova.api.ec2 result = method(context, **args)
2012-09-13 20:55:05 TRACE nova.api.ec2 File "/usr/lib/python2.7/dist-packages/nova/api/ec2/cloud.py", line 1115, in describe_addresses
2012-09-13 20:55:05 TRACE nova.api.ec2 addresses = [self._format_address(context, f) for f in floatings]
2012-09-13 20:55:05 TRACE nova.api.ec2 File "/usr/lib/python2.7/dist-packages/nova/api/ec2/cloud.py", line 1123, in _format_address
2012-09-13 20:55:05 TRACE nova.api.ec2 if fixed['instance_id'] is not None:
2012-09-13 20:55:05 TRACE nova.api.ec2 KeyError: 'instance_id'
2012-09-13 20:55:05 TRACE nova.api.ec2