mongoose: MongoDB error when adding the async cursor option to aggregate query

When adding the async cursor option I get the following error:

MongoError: cursor object can't contain fields other than batchSize

The (simplified) query:

var pipeline = [
    { $lookup: {
        from: "interactions",
        localField: "_id",
        foreignField: "message_id",
        as: "interactions"
    }},
    { $project: {
        id: "$_id",
        numberOfInteractions: { $size: "$interactions" }
    }},
    { $match: {
        numberOfInteractions: { $eq: 0 }
    }}
];

var promise = Message
    .aggregate(pipeline)
    .allowDiskUse(true)
    .cursor({ batchSize: 2500, async: true })
    .exec();

If I make this modification the error goes away:

diff --git a/lib/aggregate.js b/lib/aggregate.js
index fd2c2fc..ad76bd6 100644
--- a/lib/aggregate.js
+++ b/lib/aggregate.js
@@ -499,6 +499,7 @@ Aggregate.prototype.exec = function(callback) {

   if (this.options && this.options.cursor) {
     if (this.options.cursor.async) {
+      delete this.options.cursor.async;
       return new Promise.ES6(function(resolve) {
         if (!_this._model.collection.buffer) {
           process.nextTick(function() {

My setup:

NodeJS 4.2.4
MongoDB version 3.2.5
Mongoose 4.4.13