Docker desktop ipv6 support fixed-cidr-v6 format error

This is what I placed in the Docker engine.

  "builder": {
    "gc": {
      "defaultKeepStorage": "20GB",
      "enabled": true
  "ipv6": true,
  "fixed-cidr-v6": "fd12:3456:789a:1::/64",
  "experimental": true,
  "ip6tables": true

Why am I getting this error?

/fixed-cidr-v6 must match format "ipv6"

I’m just following this guide: all-in-one/ at main · nextcloud/all-in-one · GitHub
Docker version: v4.27.1

The value for fixed-cidr-v6 is a valid ipv6. I would have expected the problem on another level: docker desktop always runs in a utility vm (with own private network), so I would have only expected a problem to access the ULA from the host.

May I suggest posting this as issue in the Github Project for Docker Desktop for Windows: I feel the error message is a bug.