How do I remove excess layers

[zgmslm]

When I use cereal:

#include <iostream>  
#include <sstream>  
#include <cereal/types/string.hpp>  
#include <cereal/archives/json.hpp>  

struct person
{
    std::string name;
    int age;

    template <class Archive>
    void serialize(Archive & archive)
    {
        archive(CEREAL_NVP(name), CEREAL_NVP(age));
    }
};

int main()
{
    person p = { "Alice", 30 };

    std::stringstream ss;
    {
        cereal::JSONOutputArchive archive(ss);
        archive(p);
    }
 
    std::string json_string = ss.str();
    std::cout << "Serialized JSON: \n" << json_string << std::endl;

    return 0;
}

I expect to serialize to the string：
{ "name": "Alice", "age": 30 }

But the serialized string is:

{
    "value0": {
        "name": "Alice",
        "age": 30
    }
}

cereal adds an additional layer to the top object, How do I use cereal to generate json with no additional layers?