Default value ignore in substitute

[yodamad]

Hi,

I'm trying to use substitute mechanism with a default value.

Current version of flux : ghcr.io/fluxcd/kustomize-controller:v1.4.0

So I defined a Namespaceobject :

---
apiVersion: v1
kind: Namespace
metadata:
  name: ${frontend_ns_name:=dev}

And define my GitRepository and Kustomization but without overriding the default value for my default env.

---
apiVersion: source.toolkit.fluxcd.io/v1
kind: GitRepository
metadata:
  name: commons-gitops
  namespace: flux-system
spec:
  interval: 1m0s
  ref:
    branch: feat/fluxcd
  timeout: 60s
  url: https://gitlab.com/xxx/demo/manifests.git

apiVersion: kustomize.toolkit.fluxcd.io/v1
kind: Kustomization
metadata:
  name: flux-demo-frontend
  namespace: flux-system
spec:
  interval: 1m0s
  path: ./frontend
  prune: true
  sourceRef:
    kind: GitRepository
    name: commons-gitops

When Flux try to reconcile the information, it fails explaining that the value is not valid :

Reconciliation failed after 150.548712ms, next try in 1m0s","controller":"kustomization","controllerGroup":"kustomize.toolkit.fluxcd.io","controllerKind":"Kustomization","Kustomization":{"name":"flux-demo-frontend","namespace":"flux-system"},"namespace":"flux-system","name":"flux-demo-frontend","reconcileID":"5ed498e8-b838-42e1-8d82-dc6671ae7f3a","revision":"feat/fluxcd@sha1:4aa0ecf8882974ab1c8670e457f6ff6cb26544a9
[...]
"Namespace/${frontend_ns_name:=dev} dry-run failed (Invalid): Namespace \"${frontend_ns_name:=dev}\" is invalid: [metadata.name: Invalid value: \"${frontend_ns_name:=dev}\": a lowercase RFC 1123 label must consist of lower case alphanumeric characters or '-' ...

Don't know what I'm doing wrong here as I think that I follow correctly the doc sample

[yodamad]

If I add the configuration :

  postBuild:
    substitute:
      frontend_ns_name: "dev"

It works, but I suppose that default value should work without this