Inconsistent results when using MinKey/MaxKey in comparison operator when an index is present

> db.test.find()

{ "_id" : ObjectId("5aa9341ad89e0643662e2f30"), "a" : { "$minKey" : 1 } }

> db.test.getIndexes()

[

	{

		"v" : 2,

		"key" : {

			"_id" : 1

		},

		"name" : "_id_",

		"ns" : "test.test"

	}

]

> db.test.find({a: {$gt: MinKey}})

> db.test.createIndex({a: 1})

{

	"createdCollectionAutomatically" : false,

	"numIndexesBefore" : 1,

	"numIndexesAfter" : 2,

	"ok" : 1

}

> db.test.find({a: {$gt: MinKey}})

{ "_id" : ObjectId("5aa9341ad89e0643662e2f30"), "a" : { "$minKey" : 1 } }