config.User in image config

I created an oci image by following the image-spec. While converting my 'image' into a runtime-bundle using oci-image-tool convert, I received the error:

config.User: unsupported format
github.com/opencontainers/image-tools/image.runtimeSpec
        /c/Users/mattm/repo/personal/go/src/github.com/opencontainers/image-tools/image/config.go:103
github.com/opencontainers/image-tools/image.createBundle
        /c/Users/mattm/repo/personal/go/src/github.com/opencontainers/image-tools/image/image.go:342
github.com/opencontainers/image-tools/image.createRuntimeBundle
        /c/Users/mattm/repo/personal/go/src/github.com/opencontainers/image-tools/image/image.go:289
github.com/opencontainers/image-tools/image.CreateRuntimeBundleLayout
        /c/Users/mattm/repo/personal/go/src/github.com/opencontainers/image-tools/image/image.go:235
main.createAction
        /c/Users/mattm/repo/personal/go/src/github.com/opencontainers/image-tools/cmd/oci-image-tool/create.go:74
github.com/opencontainers/image-tools/vendor/github.com/urfave/cli.HandleAction
        /c/Users/mattm/repo/personal/go/src/github.com/opencontainers/image-tools/vendor/github.com/urfave/cli/app.go:485
github.com/opencontainers/image-tools/vendor/github.com/urfave/cli.Command.Run
        /c/Users/mattm/repo/personal/go/src/github.com/opencontainers/image-tools/vendor/github.com/urfave/cli/command.go:193
github.com/opencontainers/image-tools/vendor/github.com/urfave/cli.(*App).Run
        /c/Users/mattm/repo/personal/go/src/github.com/opencontainers/image-tools/vendor/github.com/urfave/cli/app.go:250
main.main
        /c/Users/mattm/repo/personal/go/src/github.com/opencontainers/image-tools/cmd/oci-image-tool/main.go:63
runtime.main
        /usr/local/go/src/runtime/proc.go:195
runtime.goexit
        /usr/local/go/src/runtime/asm_amd64.s:2337

My image config came straight from the spec's documentation with some minor changes such as the user name, home directory, etc.

{
    "schemaVersion": 2,
    "created": "2019-06-29T04:43:42.455040700Z",
    "author": "Bob T. Hacker <bobthacker@example.com>",
    "architecture": "amd64",
    "os": "linux",
    "config": {
        "User": "root",
        "ExposedPorts": {},
        "Env": [
            "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
            "FOO=oci_is_a",
            "BAR=well_written_spec"
        ],
        "Entrypoint": [
            "/bin/sh"
        ],
        "Cmd": [],
        "Volumes": {},
        "WorkingDir": "/",
        "Labels": {
            "com.example.project.git.url": "https://example.com/project.git",
            "com.example.project.git.commit": "45a939b2999782a3f005621a8d0f29aa387e1d6b"
        }
    },
    "rootfs": {
        "diff_ids": [
            "sha256:ec3da7fb5f709a1ce912e6e31fccc5588420c5f1dcecc362c72c989532c1917a"
        ],
        "type": "layers"
    },
    "history": [
        {
            "created": "2019-06-29T04:43:42.455040700Z",
            "created_by": "/bin/sh -c #(nop) CMD [\"sh\"]",
            "empty_layer": true
        }
    ]
}

The code responsible for throwing the error is on lines 87-104 in the image/config.go file

	if uid, err := strconv.Atoi(c.Config.User); err == nil {
		s.Process.User.UID = uint32(uid)
	} else if ug := strings.Split(c.Config.User, ":"); len(ug) == 2 {
		uid, err := strconv.Atoi(ug[0])
		if err != nil {
			return nil, errors.New("config.User: unsupported uid format")
		}

		gid, err := strconv.Atoi(ug[1])
		if err != nil {
			return nil, errors.New("config.User: unsupported gid format")
		}

		s.Process.User.UID = uint32(uid)
		s.Process.User.GID = uint32(gid)
	} else if c.Config.User != "" {
		return nil, errors.New("config.User: unsupported format")
	}

Going by the code, I changed the config.User value to "0:0" (i.e: uid:gid) and the command oci-image-tool convert command completed successfully.

Is the documentation wrong? Are developers not supposed to enter an actual username in the config.User field?

UPDATE
After running the runtime-bundle in docker, I get the following error:

docker: Error response from daemon: linux spec user: unable to find user 0/0: no matching entries in passwd file.

Start with the image-spec config.md documentation and image/config.go, then reproduce conversion with the shown username and 0:0 values. Compare how named users and uid:gid values are handled, including the Docker passwd-file error. Done means the specification and implementation agree, with the behavior covered by a focused test or documentation correction.