About [nova diagnostic xxx]command

I'm following the documents from Openstack to install Essex on Ubuntu server 12.04 LTS. All services work correctly.
The nova diagnostic command is a command that can obtain extra informations about the instance:its CPU usage,
the memory, the disk io or network io, per instance.
After I bulit a instance ,this command seems my deploy something wrong.But I can't figure it out.The err info:
ERROR: The server has either erred or is incapable of performing the requested operation. (HTTP 500)

Thanks!