passwd-in-image: /dev/nbd7p1 does not exist

I've only seen this on one system (Debian Stretch amd64).

$ sudo bin/passwd-in-image build/freedombox-unstable-free_2016-05-07_all-amd64.vdi fbx --password frdm
INFO:__main__:Checking for necessary dependencies
INFO:__main__:Adding partition mappings for VM disk image - build/freedombox-unstable-free_2016-05-07_all-amd64.vdi
INFO:__main__:Root device is - /dev/nbd7p1
INFO:__main__:Mounting /dev/nbd7p1 on /tmp/tmp04kylp6o
mount: special device /dev/nbd7p1 does not exist
INFO:__main__:Removing partition mappings from VM device - /dev/nbd7
ERROR:__main__:Error running command: ['mount', '/dev/nbd7p1', '/tmp/tmp04kylp6o']