Modifying user.username in Session With PassportJS

I have a form that allows people to update their profile information, which is populated with data from req.user via PassportJS.

The problem is that whenever I update the value that corresponds to user.username I get the following error message:

TypeError: Cannot read property '_id' of null 

From line 6 in this snippet of code:

passport.deserializeUser(function(id, done) {
  mongo.connect("mongodb://localhost:27017/formulas", function(e, db){
  if (e) {return next(e);}
  var col = db.collection("users");
  col.findOne({"username": id}, function(err, user){
    done(err, {"id": user._id, "username": id, "activeEmail": user.activeEmail, "name": user.name, "password": user.password, "formulas": user.formulas});
    });
  });
});

I'm assuming it's because in serializeUser I'm using user.username to load it into session like so:

passport.serializeUser(function(user, done) {
  done(null, user.username);
});

Does anyone know how to go about getting around this or is it an intractable issue with Passport?

The code I have that does the update generically looks like this:

router.post('/update-profile', function(req, res) {
    var name = req.body.name;
    var username = req.body.username;
    var db = req.db.collection('users');

    db.updateOne({"username": username}, {
      $set: {
          "name": name,
          "username": username,
        }
      }, function(err, r) {
         assert.equal(null, err);
         assert.equal(null, r.matchedCount);
      }
    });
    res.render('profile', {
      user: req.user
    });
  });

UPDATE:

Per comment request, the error message from findOne in serializeUser is null when it's called, so it's not the query that's the problem.