mongoose-ref | node v8 | Runtime Evironment library

My app works locally, but in production I can't seem to fetch a specific thing from mongo using mongoose. I've tried:

var Coach = mongoose.model('Coach', CoachSchema);

• Coach.findOne({ _id: req.session.userId}, function (err, coach) {})
• Coach.find({ _id:req.session.userId}, function (err, coach) {})
• Coach.findById(req.session.userId).exec(function(err, coach)){})

I've received the same error: Cast to ObjectId failed for value "586bc06bd09d870012baa5b8" at path "_id" for model "Coach"

Like I said, it works locally with both nodemon and heroku local but on my heroku production server it doesn't work.

I've read a little about this type of cast error here:

• What's Mongoose error Cast to ObjectId failed for value XXX at path "_id"?
• Mongoose refuses to cast valid string to ObjectId
• Cast to ObjectId failed for value "586cc8b3ea780c071bbe2469" at path "_id" for model "User"

But no luck yet. Any help would be great. Thank you in advance.