net/http not working with mux routers about go-redoc HOT 2 CLOSED

internal.HandleFunc("/openapi.yaml", HandlerReturnsOpenapiYamlContent)

func main() {
	doc := redoc.Redoc{
		Title:       "Example API",
		Description: "Example API Description",
		SpecFile:    "./openapi.json",
		SpecPath:    "/openapi.json",
		DocsPath:    "/docs",
	}

	mux := http.ServeMux{}
	handler := doc.Handler()
	mux.Handle("/docs", handler)
	mux.Handle("/openapi.json", handler)
// ...
}