Mongoose MapReduce top values

I have code similar to the example here http://mongoosejs.com/docs/api.html#model_Model.mapReduce

The main difference is that I'm trying to only save values which are within the top five.

For example, if the resulting mapReduce collection generates the following:

{
    "_id": "1",
    "value": 6
}
{
    "_id": "2",
    "value": 32
}
{
    "_id": "3",
    "value": 1
}
{
    "_id": "4",
    "value": 5
}
{
    "_id": "5",
    "value": 6
}
{
    "_id": "6",
    "value": 7
}

What would be the best method for only saving the documents which have a value key within the top five to the mapReduce results?

I've tried using the sort option but that produces this error:

[MongoError: exception: could not create cursor over [db].[collection] for query : {} sort : { value: 1 }]