Unable to create bay / Error: resources.kube_minions.resources[0].resources.docker_volume_attach - Invalid input for field/attribute device. Value: None. None is not of type 'string'

[Bertrand NOEL]

On Ubuntu 14.04, Devstack install, with Magnum from master.

I try to create a Kubernetes bay, following the instructions here:
https://github.com/openstack/magnum/blob/master/doc/source/dev/dev-quickstart.rst#exercising-the-services-using-devstack

I get the following error when the node is created:

ERROR magnum.conductor.handlers.bay_conductor [-] Unable to create bay, stack_id: 12a87dc6-f6bc-43e6-a158-39d884840f88, reason: Resource CREATE failed: Error: resources.kube_minions.resources[0].resources.docker_volume_attach: Failed to attach volume db0885d1-7584-4d17-9b09-8f869bfe9378 to server aa845ca7-c44b-42d9-bcd5-e0419ad22e43 - Invalid input for field/attribute device. Value: None. None is not of type 'string' (HTTP 400) (Request-ID: req-3c8cbc56-26d8-485d-befa-ba4939b88fed)