MongoDB: Sorting by date treats null field as earlier date

Mongoose schema includes this:

{
  dueDate: { type: Date, required: false }
}

My goal is to fetch docs sorted by dueDate with the soonest date at the top, followed by the rest with no dueDate. My problem is that a due date is not required therefore docs with no dueDate get sorted above docs with a valid date.

This is how I sort it (node.js & mongoose.js):

query.sort({ dueDate: 'asc' });

I'm mainly looking for a built in way to do this; I've already implemented a hack by combining queries.