Mongoose sort by date doing nothing

The following function fetches a collection from my database:

module.exports.getUserSubscriptions = function(id, callback) {
    Subscription.find({user:id})
    .sort({start_date: 'desc'})
    .populate({
        path:'plan',
        model:'Plan'
    })
    .lean()
    .exec(callback);
}

Regardless of how I call sort, I can't make it actually sort the data.

start_date is defined in the Subscription Schema like this:

start_date: {
    type: Date,
    default: Date.now
}

and looks like this when queried:

ISODate("2017-05-16T11:36:58.605Z")

I've tried adding the sort to the find query too with no effect. I've also tried using .sort("-start_date") but nothing I try makes any difference to the output.

Any suggestions would be greatly appreciated.

via Taintedmedialtd