Mongoose - Query nested document by string

I've got the following schema:

var invitationSchema = new Schema({
    emailAddress : String
});

var eventSchema = new Schema({
    name : String,
    start_date : Date,
    end_date : Date,
    venue : { type : Schema.ObjectId, ref : 'Venue' },
    invitees : [invitationSchema]
});

Which seems fine to me...?
I'm trying to do the following query:

Events.findOne({'invitees._id' :'4f8dcb06ee21783d74000040'}, function(err, myEvent) {
    //myEvent is null?
});

myEvent is null, even though, in my Events collection, there is an Event doc, with an array of invitees, one of those, has an id of 4f8dcb06ee21783d74000040.

I read this, which says it should now work?