helm: 'lookup' function always returns empty map - kubernetes-helm

Question

The relevant docs: https://helm.sh/docs/chart_template_guide/functions_and_pipelines/#using-the-lookup-function
My helm version:
$ helm version
version.BuildInfo{Version:"v3.4.1",
GitCommit:"c4e74854886b2efe3321e185578e6db9be0a6e29",
GitTreeState:"dirty", GoVersion:"go1.15.4"}
Minimal example to reproduce:
Create a new helm chart and install it.
$ helm create my-chart
$ helm install my-chart ./my-chart
Create a simple ConfigMap.
# my-chart/templates/configmap.yaml
apiVersion: v1
kind: ConfigMap
metadata:
name: my-configmap
data:
someKey: someValue
Upgrade the existing chart so that the ConfigMap is applied.
$ helm upgrade my-chart ./my-chart
Confirm that the ConfigMap exists.
$ kubectl -n default get configmap my-configmap
Which returns as expected:
NAME DATA AGE
my-configmap 1 12m
Try to use the lookup function to reference the existing ConfigMap.
# my-chart/templates/configmap.yaml
apiVersion: v1
kind: ConfigMap
metadata:
name: my-configmap
data:
someKey: someValue
someOtherKey: {{ (lookup "v1" "ConfigMap" "default" "my-configmap").data.someValue }}
Then do a dry-run of the upgrade.
$ helm upgrade my-chart ./my-chart --dry-run
You will be met with a nil pointer error:
Error: UPGRADE FAILED: template: my-chart/templates/configmap.yaml:9:54: executing "my-
chart/templates/configmap.yaml" at <"my-configmap">: nil pointer evaluating interface
{}.someValue
What am I doing wrong?

Answer 1

This is an expected behavior if you are using --dry-run flag.
From documentation
Keep in mind that Helm is not supposed to contact the Kubernetes API
Server during a helm template or a helm install|update|delete|rollback --dry-run, so the lookup function will return an empty list (i.e. dict) in such a case.