The error in vdsm.log

Traceback (most recent call last):
  File "/usr/share/vdsm/virt/vm.py", line 2016, in _setup_devices
    dev_object.setup()
  File "/usr/lib/python2.7/site-packages/vdsm/virt/vmdevices/graphics.py", line 63, in setup
    net_api.create_libvirt_network(display_network, self.conf['vmId'])
  File "/usr/lib/python2.7/site-packages/vdsm/network/api.py", line 90, in create_libvirt_network
    libvirt.create_network(netname, user_reference)
  File "/usr/lib/python2.7/site-packages/vdsm/network/libvirt.py", line 94, in create_network
    if not is_libvirt_network(netname):
  File "/usr/lib/python2.7/site-packages/vdsm/network/libvirt.py", line 159, in is_libvirt_network
    netname = LIBVIRT_NET_PREFIX + netname
TypeError: cannot concatenate 'str' and 'NoneType' objects
2017-03-29 22:58:39,559-0400 ERROR (vm/d71bdf4e) [virt.vm] (vmId='d71bdf4e-1eb3-4762-bd0e-05bb9f5e43ef') The vm start process failed (vm:659)

The tests last passed on Mar 28. Did a recent patch break this?