create cluster-manifests fails when imageContentSources is missing

$ openshift-install agent create cluster-manifests
INFO Consuming Install Config from target directory
FATAL failed to write asset (Mirror Registries Config) to disk: failed to write file: open .: is a directory

apiVersion: v1alpha1
metadata:
  name: appliance
rendezvousIP: 192.168.122.116
hosts:
  - hostname: sno
    installerArgs: '["--save-partlabel", "agent*", "--save-partlabel", "rhcos-*"]'
    interfaces:
     - name: enp1s0
       macAddress: 52:54:00:e7:05:72
    networkConfig:
      interfaces:
        - name: enp1s0
          type: ethernet
          state: up
          mac-address: 52:54:00:e7:05:72
          ipv4:
            enabled: true
            dhcp: true