Mongoose idGetter error: Cannot set property '_id' of null

I am getting the following error from the Mongoose idGetter function. Can anyone shed some light on what may be causing it?

  return this.$__._id = null == this._id
                  ^
TypeError: Cannot set property '_id' of null
    at model.idGetter (e:\Users\Christine\test\mvt\node_modules\mongoose\lib\schema.js:98:23)

The error situation involves multiple client requests performing document removes (on different documents in the same collection), on a database being accessed over internet (i.e. slow response time). The failure happens on accessing the id of one of the removed documents. There appears to be a small timing window in which it occurs - everything I've tried to debug it causes the problem to 'go away'.

Outline of the application code:

Details.findOneAndRemove({ _id : remove_id}, afterRemove);

function afterRemove(err, removedDoc){
  if (!err && removedDoc){
    OtherDetails.find({ _id: some_id}, afterFind);
  }

  function afterFind(err, foundDoc){
    if (!err && foundDoc){
      // next line causes exception
      if (foundDoc.details_id === removedDoc.id) { // do stuff };
    }
  }
}