kubernetes: api server's ValidateCustomResource failed for map with additionalProperties of string type

What happened:

Trying to use k8s.io/apiextensions-apiserver/pkg/apiserver/validation to validate a YAML file with OpenAPI.

The YAML file contains a map:

labels:
    a: b

And the openAPIV3Schema generated by kubebuilder contains below logic:

                      labels:
                        additionalProperties:
                          type: string
                        description: 'xxx.'
                        type: object

When I use this openAPIV3Schema to verify my YAML file, it will have errors:

unexpected validation error for map[field:map[a:b]]: [field: Invalid value: "a": field.a in body is a forbidden property]

What you expected to happen:

It should not throw error

How to reproduce it (as minimally and precisely as possible):

I wrote a unit test: https://github.com/kubernetes/apiextensions-apiserver/pull/52/files, this test is failing.

Anything else we need to know?:

Environment:

• Kubernetes version (use kubectl version): 1.20
• Cloud provider or hardware configuration: None.
• OS (e.g: cat /etc/os-release):
• Kernel (e.g. uname -a):
• Install tools:
• Network plugin and version (if this is a network-related bug):
• Others: