Google appengine / ah / health“方法不存在。”

Requests to projectname.appspot.com/_ah/health return the following error

{
 "code": 5,
 "message": "Method does not exist.",
 "details": [
  {
   "@type": "type.googleapis.com/google.rpc.DebugInfo",
   "stackEntries": [],
   "detail": "service_control"
  }
 ]
}

Tested locally and it works fine.

abbreviated app.yaml:

runtime: go
env: flex
api_version: go1

app.go main function:

func main() {

    r := mux.NewRouter()

    r.HandleFunc("/", handler)

    r.HandleFunc("/_ah/health", healthCheckHandler)

    http.Handle("/", r)

    port := 8080
    if portStr := os.Getenv("PORT"); portStr != "" {
        port, _ = strconv.Atoi(portStr)
    }
    log.Fatal(http.ListenAndServe(fmt.Sprintf(":%d", port), nil))
}

Any idea's what's going wrong?

Add this to open API

paths:
  "/_ah/health":
    get:
      operationId: "OkStatus"
      security: []
      responses:
        200:
          description: "Ok message"

It's not let through the ESP proxy