Dapper QueryMultiple() method returns null from postgreSQL

So I'm trying to execute pl/pgSQL functions from a web app using Dapper micro-ORM. I'm always getting null values irrespective of the values I'm passing as parameters in QueryMultiple method; although in database the function gives proper result.

DAL Method:

public List<JSONData> ExecutePostgres(string spName, List<QueryParam> Params)
    {
        List<JSONData> students = new List<JSONData>();

        using (var dbInstance = new NpgsqlConnection(postgreConnectionString))
        {
            using (var reader = dbInstance.QueryMultiple(spName, new { std_id = Params[0].value }, commandType: CommandType.StoredProcedure))
            {
                students = reader.Read<JSONData>().ToList();
            }
        }

        return students;
    }

Implementation Method:

public List<string> ExecuteProcedure(string spName, List<QueryParam> Params)
    {
        var list = objDLOperation.ExecutePostgres(spName, Params);
        var strList = new List<string>();

        list.ForEach(x => strList.Add(x.JSONResult));

        return strList;

    }

This strList is always a List of null values

Entity Class:

    public class JSONData
    {
      public string JSONResult { get; set; }
    }

pl/pgSQL Function:

CREATE OR REPLACE FUNCTION public.usp_getstudent(std_id integer)
RETURNS SETOF json 
AS $function$

BEGIN
RETURN QUERY
select row_to_json(result) from (select * from student where id = std_id) as result;
END  

$function$ LANGUAGE 'plpgsql';