Comment 1 for bug 1575622

Can you confirm that you have ControlPlaneSubnetCidr as a string e.g '24' not 24 in the environment?

This looks similar to issues fixed via https://review.openstack.org/#/c/299664/ - your analysis is different, but I saw a similar error message because ip_subnet resolves to null in the output which consumes the non-string subnet.