$ positional operator with $elemMatch doesn't work when multiple update critera supplied

db.test.insert({array1: ["a", "b"], array2:[{_id:1}, {_id:2}]})

db.test.update({array1: "a", array2:{$elemMatch:{_id:2}}}, {$set {"array2.$.flag":"THIS ELEMENT WAS UPDATED"}});

{ "_id" : ObjectId("54b84eb0d9f8df727c34dda5"), "array1" : [  "a",  "b" ], "array2" : [  {  "_id" : 1 },  {  "_id" : 2,  "flag" : "THIS ELEMENT WAS UPDATED" } ] }

{ "_id" : ObjectId("54b84dfa0ed2d2cc8aa29721"), "array1" : [ "a", "b" ], "array2" : [ { "_id" : 1, "flag" : "THIS ELEMENT WAS UPDATED" }, { "_id" : 2 } ] }