gqlgen: Could not connect to websocket

Could not connect to websocket endpoint wss://127.0.0.1:4000/api/query. Please check if the endpoint url is correct.

Im using mux.

resolver.go

var TicketSub struct {
	sync.RWMutex
	Subscribers map[string]chan *gqlapi.TicketStatusChangePayload
}

func (r *subscriptionRootResolver) TicketStatusChanged(ctx context.Context) (<-chan *gqlapi.TicketStatusChangePayload, error) {
	user := &models.User{
		UUID: "e70e78bb-9d08-405d-a0ed-266ec703de19",
	}
	events := make(chan *gqlapi.TicketStatusChangePayload, 1)

	go func() {
		<-ctx.Done()
		TicketSub.Lock()
		delete(TicketSub.Subscribers, user.UUID)
		TicketSub.Unlock()
	}()

	TicketSub.Lock()
	TicketSub.Subscribers[user.UUID] = events
	TicketSub.Unlock()

	return events, nil
}

func (r *queryRootResolver) SubscriptionTest(ctx context.Context) (err error) {
	id := uuid.New().String()
	notifyTicketStatusChange("e70e78bb-9d08-405d-a0ed-266ec703de19", id, fmt.Sprintf("Ticket (uuid=%s) status changed.", id))
	return
}

main.go

func main() {
	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
	Start(ctx)

	// Wait for interrupt signal to gracefully shutdown the server with
	// a timeout of 10 seconds.
	sigs := make(chan os.Signal, 1)
	signal.Notify(sigs,
		os.Kill,
		os.Interrupt,
		syscall.SIGHUP,
		syscall.SIGINT,
		syscall.SIGTERM,
		syscall.SIGQUIT)
	<-sigs

	defer func() {
		cancel()
	}()

}

func Start(ctx context.Context) {
	go start(ctx)
}

func start(ctx context.Context) {
	cfg := gqlapi.Config{
		Resolvers: &resolvers.Resolver{},
		Directives: gqlapi.DirectiveRoot{
			Auth:    directives.Auth,
		},
	}
	r := mux.NewRouter()
	r.HandleFunc("/", handler.Playground("GraphQL playground", "/api/query"))

	upgrader := websocket.Upgrader{
		CheckOrigin: func(r *http.Request) bool {
			return true
		},
		EnableCompression: true,
	}

	options := []handler.Option{
		handler.RecoverFunc(func(ctx context.Context, err interface{}) error {
			// notify bug tracker...
			return fmt.Errorf("Internel server error.")
		}),
		handler.WebsocketUpgrader(upgrader),
	}

	r.HandleFunc("/api/query", handler.GraphQL(
		gqlapi.NewExecutableSchema(cfg),
		options...,
	))
	srv := &http.Server{
		Handler: r,              //
		Addr:    "0.0.0.0:4000", //
	}
	srv.SetKeepAlivesEnabled(true)
	log.Fatal(srv.ListenAndServeTLS("cert.pem", "key.pem"))
}

I can push message via websocket tunnel as you can see on the attached screenshot, but it closed after about 10-15 seconds.