Couldn't run instance due to "negative shift count" ValueError

Hello, I couldn't run instance due to the following error.

(nova.virt.libvirt_conn
2011.1-workspace:tarmac-20110115022500-fcgl1yga6csc221i): DEBUG [N/A]
instance instance-00000001: starting toXML method from MainProcess
(pid=15753) to_xml
/usr/lib/pymodules/python2.6/nova/virt/libvirt_conn.py:650
(nova.exception
2011.1-workspace:tarmac-20110115022500-fcgl1yga6csc221i): ERROR [N/A]
Uncaught exception
(nova.exception): TRACE: Traceback (most recent call last):
(nova.exception): TRACE: File
"/usr/lib/pymodules/python2.6/nova/exception.py", line 86, in _wrap
(nova.exception): TRACE: return f(*args, **kw)
(nova.exception): TRACE: File
"/usr/lib/pymodules/python2.6/nova/virt/libvirt_conn.py", line 383, in
spawn
(nova.exception): TRACE: xml = self.to_xml(instance)
(nova.exception): TRACE: File
"/usr/lib/pymodules/python2.6/nova/virt/libvirt_conn.py", line 667, in
to_xml
(nova.exception): TRACE: network['cidr_v6'])
(nova.exception): TRACE: File
"/usr/lib/pymodules/python2.6/nova/virt/libvirt_conn.py", line 134, in
_get_net_and_prefixlen
(nova.exception): TRACE: net = IPy.IP(cidr)
(nova.exception): TRACE: File "/usr/lib/pymodules/python2.6/IPy.py",
line 202, in __init__
(nova.exception): TRACE: self._prefixlen, self._ipversion):
(nova.exception): TRACE: File "/usr/lib/pymodules/python2.6/IPy.py",
line 1266, in _checkNetaddrWorksWithPrefixlen
(nova.exception): TRACE: if net & _prefixlenToNetmask(prefixlen,
version) == net:
(nova.exception): TRACE: File "/usr/lib/pymodules/python2.6/IPy.py",
line 1295, in _prefixlenToNetmask
(nova.exception): TRACE: return ((2L<<prefixlen-1)-1) <<
(_ipVersionToLen(version) - prefixlen)
(nova.exception): TRACE: ValueError: negative shift count

And the version I used is 2011.1~bzr569-0ubuntu0ppa1~maverick3.

Please give me some advices.

Thanks.