how to make multiple requests in pqxx?

Question

I'm trying to make some operations to the database, but an error occurs terminate called after throwing an instance of 'pqxx::usage_error' what(): Started nontransaction while nontransaction still active. What could be the problem?

  pqxx::nontransaction N1(*connection);
  std::string messageid_req = "select max(messageid) + 1 from server.message";
  pqxx::result messageid_res = N1.exec((char*)messageid_req.c_str());
  int messageid = messageid_res.begin()[0].as<int>();

  pqxx::nontransaction N2(*connection);
  std::string index_in_chat_req = "select max(index_in_char) + 1 from server.message";
  pqxx::result index_in_chat_res = N2.exec((char*)index_in_chat_req.c_str());
  int index_in_chat = index_in_chat_res.begin()[0].as<int>();

  std::string insert = "insert into server.message (messageid, chatid, userid, index_in_char, text)\n";
  insert += "values (" + std::to_string(messageid) + ", " + std::to_string(chatid) + ", " + std::to_string(userid) + ", " + std::to_string(index_in_chat) + text;
  pqxx::work W(*connection);
  W.exec(insert.c_str());