waterline issue with null foreign keys

PersonController.js

  index: function(req, res) {
    Person
      .find()
      .populate('user')
      .exec(function peopleFound(err, people) {
        console.log(err);
        if (err) return res.json(err, 400);
        if (!people) return res.json({ error: 'No people exist.' }, 404);
        res.json(people);
      });
  },

UserController.js

  destroy: function(req, res) {
    var username = req.param('username');
    User.findOne({ username: req.param('username') }, function foundUser(err, user) {
      if (err) return res.json(err, 400);
      Person.update({user: user.id}, {user: null}, function profileUpdated(err, profile) {
        if (err) return res.json(err, 400);
      });
      User.destroy(user.id, function userDestroyed(err) {
        if (err) return res.json(err, 400);
        return res.json({msg: 'User '+username+' deleted.'}, 200);
      });
    });
  }