Creating Index with invalid properties

db.test.save({name : 'mongodb', date: new Date()}) WriteResult({ "nInserted" : 1 }) > db.test.save({name : 'mongodb2'}) WriteResult({ "nInserted" : 1 }) > db.test.find() { "_id" : ObjectId("55f8762792a0ee661e65b90b"), "name" : "mongodb", "date" : ISODate("2015-09-15T19:48:55.803Z") } { "_id" : ObjectId("55f8763192a0ee661e65b90c"), "name" : "mongodb2" } > db.test.ensureIndex({date :1}, {sparsse : true}) { "createdCollectionAutomatically" : false, "numIndexesBefore" : 1, "numIndexesAfter" : 2, "ok" : 1 } > db.test.getIndexes() [ { "v" : 1, "key" : { "_id" : 1 }, "name" : "_id_", "ns" : "easytaxi_driver_restore.test" }, { "v" : 1, "key" : { "date" : 1 }, "name" : "date_1", "ns" : "easytaxi_driver_restore.test", "sparsse" : true } ]