mimirtool alertmanager verify: found character that cannot start any token

[toby181]

Describe the bug

We're evaluating the mimirtool to verify / load the configuration and notification templates for the Alertmanager, using examples that are provided in the documentation.
Doing so for the provided Alertmanager config works, doing the same for the provided notification policy fails with the exception "mimirtool: error: yaml: line 2: found character that cannot start any token, try --help".

Trying some other template will mostly result in "line 1: cannot unmarshal !!map into string".

To Reproduce

Steps to reproduce the behavior:

Verifying the Alertmanager configuration works, having this:

alertmanager_config.yaml:

route:
  receiver: "example_receiver"
  group_by: ["example_groupby"]
receivers:
  - name: "example_receiver"

Verification works (no exception)

$ mimirtool alertmanager verify alertmanager_config.yaml --log.level="debug"
INFO[0000] log level set to debug                       

Doing the same for the notification policy.
alertmanager_template.tpl:

{{ define "alert_customer_env_message" }}
  [{{ .CommonLabels.alertname }} | {{ .CommonLabels.customer }} | {{ .CommonLabels.environment }}]
{{ end }}

Verification fails

$ mimirtool alertmanager verify alertmanager_template.tpl --log.level="debug"
INFO[0000] log level set to debug                       
mimirtool: error: yaml: line 2: found character that cannot start any token, try --help

Expected behavior

Either the documentation got a faulty example or there's some issue with the verification / linting.

Environment

Mimirtool is running in the latest version. Mimir itself is not involved yet.

mimirtool version  
Mimirtool, version 2.13.0 (branch: HEAD, revision: 4775ec156)
  go version:       go1.22.5
  platform:         darwin/arm64
checking latest version... You are on the latest version

Additional Context