Cast to ObjectId failed for value "586bcefdab008f00128b720d" at path "_id" for model "Coach"

I just launched a javascript app to heroku that I made following the Javascript authentication course and the error Cast to ObjectId failed for value "586bcefdab008f00128b720d" at path "_id" for model "Coach" is displaying instead of one of my routes:

  Coach.find( { _id:req.session.userId}, function (err, coach) {
    if (err) {
      next(err)
    }
    Athlete.find({ coachID: req.session.userId }, function (err, athlete) {
      if (err) {
        next(err)
      } else {
        return res.render('dashboard', {
          title: 'Dashboard',
          name: coach[0].name,
          userEmail: coach[0].email,
          exerciseArray: coach[0].exerciseArray,
          athleteArray: athlete
        });
      }
    })
  })

• Coach and Athlete are two of my mongoose/mongo collections
• The whole heroku app is crashing with logs indicating coach is not defined
• Other routes I have set up and manually navigate to indicate the database is connected properly. I am able to add and delete things from other collections. These working routes pass an empty object for the id parameter (Workout.find({}, function(err, docs){})
• The app fully runs locally with both heroku local and nodemon

I've tried:

1. Putting the key to the mongoose query in quotes. (Like {"_id":req.session.userId}) and without.
2. The alternative Coach.findById(req.session.userId).exec(function(err, coach)) { ... this gives me the same casting error.
3. Following the instructions found here on stack overflow: http://stackoverflow.com/questions/14940660/whats-mongoose-error-cast-to-objectid-failed-for-value-xxx-at-path-id