xenapi resize fails on image with external ramdisk and kernel

When performing a resize on an image with an external ramdisk and kernel I get the following error:

2012-03-22 12:42:14 DEBUG nova.virt.xenapi.vm_utils [req-7993ffbf-2d3a-4f1f-a6c2-fb13a80281f8 eeba598c2b194b6a87f933dde8a93d61 d725657ad3aa43a2998c1e19ee55ddb4] Detected DISK format for image 65c92be4-84dc-49a6-a154-2b04514ccbf0 from (pid=26827) determine_disk_image_type /opt/stack/nova/nova/virt/xenapi/vm_utils.py:944
2012-03-22 12:42:14 DEBUG nova.virt.xenapi.vm_utils [req-7993ffbf-2d3a-4f1f-a6c2-fb13a80281f8 eeba598c2b194b6a87f933dde8a93d61 d725657ad3aa43a2998c1e19ee55ddb4] Created VM instance-0000000b... from (pid=26827) create_vm /opt/stack/nova/nova/virt/xenapi/vm_utils.py:218
2012-03-22 12:42:14 ERROR nova.compute.manager [req-7993ffbf-2d3a-4f1f-a6c2-fb13a80281f8 eeba598c2b194b6a87f933dde8a93d61 d725657ad3aa43a2998c1e19ee55ddb4] ['FIELD_TYPE_ERROR', 'pV_kernel']. Setting instance vm_state to ERROR
2012-03-22 12:42:14 ERROR nova.rpc.amqp [req-7993ffbf-2d3a-4f1f-a6c2-fb13a80281f8 eeba598c2b194b6a87f933dde8a93d61 d725657ad3aa43a2998c1e19ee55ddb4] Exception during message handling
...
(nova.rpc.amqp): TRACE: vm_ref = session.call_xenapi('VM.create', rec)
(nova.rpc.amqp): TRACE: File "/opt/stack/nova/nova/virt/xenapi_conn.py", line 574, in call_xenapi
(nova.rpc.amqp): TRACE: return tpool.execute(f, *args)
(nova.rpc.amqp): TRACE: File "/usr/lib/python2.7/dist-packages/eventlet/tpool.py", line 76, in tworker
(nova.rpc.amqp): TRACE: rv = meth(*args,**kwargs)
(nova.rpc.amqp): TRACE: File "/usr/local/lib/python2.7/dist-packages/XenAPI.py", line 229, in __call__
(nova.rpc.amqp): TRACE: return self.__send(self.__name, args)
(nova.rpc.amqp): TRACE: File "/usr/local/lib/python2.7/dist-packages/XenAPI.py", line 133, in xenapi_request
(nova.rpc.amqp): TRACE: result = _parse_result(getattr(self, methodname)(*full_params))
(nova.rpc.amqp): TRACE: File "/usr/local/lib/python2.7/dist-packages/XenAPI.py", line 203, in _parse_result
(nova.rpc.amqp): TRACE: raise Failure(result['ErrorDescription'])
(nova.rpc.amqp): TRACE: Failure: ['FIELD_TYPE_ERROR', 'pV_kernel']

This might be as designed, but:
 * I couldn't (quickly) find any docs to confirm this
 * it should really fail before attempting the resize, if it knows it will not work