setting key beging "a" for platform.gcp.userLabels got error message which doesn't explain what's wrong exactly

4.14.0-0.nightly-2023-10-15-164249

Always

1. "create install-config"
2. edit the install-config.yaml to insert userLabels settings (see [1])
3. "create cluster" 

Error message shows up telling the label key "a" is invalid.

There should be no error, according to the statement "A label key can have a maximum of 63 characters and cannot be empty. Label must begin with a lowercase letter, and must contain only lowercase letters, numeric characters, and the following special characters `_-`".

$ openshift-install version
openshift-install 4.14.0-0.nightly-2023-10-15-164249
built from commit 359866f9f6d8c86e566b0aea7506dad22f59d860
release image registry.ci.openshift.org/ocp/release@sha256:3c5976a39479e11395334f1705dbd3b56580cd1dcbd514a34d9c796b0a0d9f8e
release architecture amd64
$ openshift-install explain installconfig.platform.gcp.userLabels
KIND:     InstallConfig
VERSION:  v1

RESOURCE: <[]object>
  userLabels has additional keys and values that the installer will add as labels to all resources that it creates on GCP. Resources created by the cluster itself may not include these labels. This is a TechPreview feature and requires setting CustomNoUpgrade featureSet with GCPLabelsTags featureGate enabled or TechPreviewNoUpgrade featureSet to configure labels.

FIELDS:
    key <string> -required-
      key is the key part of the label. A label key can have a maximum of 63 characters and cannot be empty. Label must begin with a lowercase letter, and must contain only lowercase letters, numeric characters, and the following special characters `_-`.    value <string> -required-
      value is the value part of the label. A label value can have a maximum of 63 characters and cannot be empty. Value must contain only lowercase letters, numeric characters, and the following special characters `_-`.

$ 

[1]
$ yq-3.3.0 r test12/install-config.yaml platform
gcp:
  projectID: openshift-qe
  region: us-central1
  userLabels:
  - key: createdby
    value: installer-qe
  - key: a
    value: hello
$ yq-3.3.0 r test12/install-config.yaml featureSet
TechPreviewNoUpgrade
$ yq-3.3.0 r test12/install-config.yaml credentialsMode
Passthrough
$ openshift-install create cluster --dir test12
ERROR failed to fetch Metadata: failed to load asset "Install Config": failed to create install config: invalid "install-config.yaml" file: platform.gcp.userLabels[a]: Invalid value: "hello": label key is invalid or contains invalid characters. Label key can have a maximum of 63 characters and cannot be empty. Label key must begin with a lowercase letter, and must contain only lowercase letters, numeric characters, and the following special characters `_-` 
$