[libvirt-users] error : Failed to find user record for uid '32722'

I'm trying to use the libvirt Python API to manage VirtualBox VMs on Mac OS X 10.8.3. I'm running libvirt 1.0.6 via Homebrew. However, the following trivial Python script: import libvirt conn = libvirt.open('vbox:///session') gives me the following error: libvirt: error : Failed to find user record for uid '32722': Success Traceback (most recent call last): File "ch3vm.py", line 3, in <module> conn = libvirt.open('vbox:///session') File "/usr/local/lib/python2.7/site-packages/libvirt.py", line 236, in open if ret is None:raise libvirtError('virConnectOpen() failed') libvirt.libvirtError: Failed to find user record for uid '32722': Success I have no idea where that uid 32722 is coming from -- it's not the uid of my user account, any of my VirtualBox files, or anything else that I'm aware of. Anyone have any idea what's going on here?