RangeError: Maximum call stack size exceeded при сохранении нескольких схем в Mongoose?

Всем привет!
Реализую ф-онал с добавлением "друга" в схему Mongoose. Вот код на POST-route :

Router.route("/confirm_friend").post(function(req, res) {
  UserSchema.findOne({ _id: req.body.self }).then(
    (self) => {
      UserSchema.findOne({ _id: req.body.whom }).then(
        (whom) => {
          self.friends.push(whom)
          whom.friends.push(self);
          self.requests.splice(self.requests.indexOf(whom), 1);
          self.save();
          whom.save();
          res.send("success");
        },
        (err) => {
          res.send(err);
          console.log("--- err", err);
        },
      );
    },
    (err) => {
      console.log("--- err", err);
    },
  );
});

Как видно из кода, я получаю две id, self и whom, на их основе ищу юзеров в базе, удаляю элемент из requests и добавляю в friends. Увы, получаю данную ошибку :

function cloneObject(obj, options) {
^

RangeError: Maximum call stack size exceeded
at cloneObject (/Users/shchypylov/Documents/projects/personal/social/node_modules/mongoose/lib/utils.js:249:21)
at clone (/Users/shchypylov/Documents/projects/personal/social/node_modules/mongoose/lib/utils.js:183:16)
at cloneObject (/Users/shchypylov/Documents/projects/personal/social/node_modules/mongoose/lib/utils.js:257:11)
at clone (/Users/shchypylov/Documents/projects/personal/social/node_modules/mongoose/lib/utils.js:183:16)
at cloneObject (/Users/shchypylov/Documents/projects/personal/social/node_modules/mongoose/lib/utils.js:257:11)
at clone (/Users/shchypylov/Documents/projects/personal/social/node_modules/mongoose/lib/utils.js:183:16)
at cloneObject (/Users/shchypylov/Documents/projects/personal/social/node_modules/mongoose/lib/utils.js:257:11)
at clone (/Users/shchypylov/Documents/projects/personal/social/node_modules/mongoose/lib/utils.js:183:16)
at cloneObject (/Users/shchypylov/Documents/projects/personal/social/node_modules/mongoose/lib/utils.js:257:11)
at clone (/Users/shchypylov/Documents/projects/personal/social/node_modules/mongoose/lib/utils.js:183:16)
at cloneObject (/Users/shchypylov/Documents/projects/personal/social/node_modules/mongoose/lib/utils.js:257:11)
at clone (/Users/shchypylov/Documents/projects/personal/social/node_modules/mongoose/lib/utils.js:183:16)
at cloneObject (/Users/shchypylov/Documents/projects/personal/social/node_modules/mongoose/lib/utils.js:257:11)
at clone (/Users/shchypylov/Documents/projects/personal/social/node_modules/mongoose/lib/utils.js:183:16)
at cloneObject (/Users/shchypylov/Documents/projects/personal/social/node_modules/mongoose/lib/utils.js:257:11)
at clone (/Users/shchypylov/Documents/projects/personal/social/node_modules/mongoose/lib/utils.js:183:16)

Как это лечить?
Спасибо