Finding mongoDB documents by a Boolean attribute via mongoose

Why can't I find documents from my MongoDB via mongoose if the attribute is a Boolean?

My Model:

var PerformerSchema = new Schema({
    _id: Number,
    firstname: String,
    lastname: String,
    active: Boolean
});

My Controller / Search Query:

Performer.find({
    'active': true
}, function(err, performers) {
    if (err) {
        return handleError(res, err);
    }
    if (!performers) {
        return res.send(404);
    }
    return res.json(performers);
});

This way the returned 'performers' array is empty. But if I change the type of the 'active' attribute in the model from Boolean to String the same Search Query finds the correct documents from my Mongo collection.