MongoDB multiple sort fields

gistfile1.txt

> db.users.ensureIndex({instance_id: 1, first_name: 1, last_name: 1});
> db.users.find({ instance_id: ObjectId("4f22b9cf7e298452c5000001") }).sort({first_name: 1, last_name: 1}).explain()
{
	"cursor" : "BtreeCursor instance_id_1_first_name_1_last_name_1",
	"nscanned" : 2,
	"nscannedObjects" : 2,
	"n" : 2,
	"millis" : 0,
	"nYields" : 0,
	"nChunkSkips" : 0,
	"isMultiKey" : false,
	"indexOnly" : false,
	"indexBounds" : {
		"instance_id" : [
			[
				ObjectId("4f22b9cf7e298452c5000001"),
				ObjectId("4f22b9cf7e298452c5000001")
			]
		],
		"first_name" : [
			[
				{
					"$minElement" : 1
				},
				{
					"$maxElement" : 1
				}
			]
		],
		"last_name" : [
			[
				{
					"$minElement" : 1
				},
				{
					"$maxElement" : 1
				}
			]
		]
	}
}