Juju add-k8s results in "empty controller name not valid"

I ran these commands to deploy CDK onto AWS:

juju bootstrap aws/us-east-1 cdkkf
juju deploy cs:bundle/canonical-kubernetes --trust --channel edge --overlay overlays/cdk.yml --overlay overlays/cdk-aws.yml --overlay overlays/cdk-gpu.yml
juju wait -wv
juju kubectl apply -f storage/aws-ebs.yml
juju scp -m cdkkf:default kubernetes-master/0:~/config /tmp/tmp1a3xfi1k
KUBECONFIG=/tmp/tmp1a3xfi1k juju add-k8s cdkkf -c cdkkf --region=aws/us-east-1 --storage juju-operator-storage

That resulted in this error:

ERROR empty controller name not valid

The overlays and snap juju version I'm using:

# overlays/cdk.yml
applications:
  kubernetes-master:
    options:
      channel: 1.15/stable
  kubernetes-worker:
    constraints: cores=4 mem=4G root-disk=100G
    options:
      channel: 1.15/stable

# overlays/cdk-aws.yml
description: Charmed Kubernetes overlay to add native AWS support.
applications:
  aws-integrator:
    annotations:
      gui-x: "600"
      gui-y: "300"
    charm: cs:~containers/aws-integrator
    num_units: 1
    trust: true
relations:
  - ['aws-integrator', 'kubernetes-master']
  - ['aws-integrator', 'kubernetes-worker']

# overlays/cdk-gpu.yml
applications:
  kubernetes-worker:
    to: ["0"]
machines:
  0:
    constraints: root-disk=100G instance-type=p2.xlarge

# storage/aws-ebs.yml
kind: StorageClass
apiVersion: storage.k8s.io/v1
metadata:
  name: k8s-ebs
  annotations:
    storageclass.kubernetes.io/is-default-class: "true"
parameters:
  type: gp2
provisioner: kubernetes.io/aws-ebs
reclaimPolicy: Delete
volumeBindingMode: WaitForFirstConsumer

# snap info juju
...
installed: 2.7-beta1+develop-df808f2 (9063) 75MB classic