why this Singleton in c++11 does't work well?

template<typename T>
class Singleton
{
    public:
    Singleton() = default;
    ~Singleton() = default;
    //forbid copy and asign
    Singleton(const Singleton &) = delete;
    Singleton&operator=(const Singleton&) = delete;

    //make singleton instance by args
    template <typename...Args>
    static void makeInstance(Args&&...args)
    {
        std::call_once(flag, make_shared_instance<T>, std::forward<Args>(args)...);
    }

    //get instance
    static std::shared_ptr<T> Instance()
    {
        if (!instance)
        {
            throw std::exception("instance not make!");
        }

        return instance;
    }

private:
    template <typename...Args>
    static void make_shared_instance(Args&&...args)
    {
        instance = std::make_shared<T>(std::forward<Args>(args)...);
    }

    static std::once_flag flag;
    static std::shared_ptr<T> instance;
};

when I use it like this : Singleton::makeInstance(10); which with parameters it works well but
Singleton::makeInstance(); which without parameters it works not well. this is why? should one help me ?