MongoDB: findandmodify seems to use the wrong query on the update command

gistfile1.js

> db.runCommand({ findAndModify: 'assets', query: {$or: [ { "_id" : "4ff70c97c87a97bc7f000002" }, { "_id" : ObjectId("4ff70c97c87a97bc7f000002") } ]}, update: {$set: {private: true}}, new: true, upsert: false })
{
	"lastErrorObject" : {
		"updatedExisting" : true,
		"n" : 1,
		"connectionId" : 1317,
		"err" : null,
		"ok" : 1
	},
	"value" : {
REDACTED
	}
}


> db.system.profile.find().forEach(printjson)
{
	"ts" : ISODate("2012-08-15T02:51:43.240Z"),
	"op" : "query",
	"ns" : "gimmebar.assets",
	"query" : {
		"$or" : [
			{
				"_id" : "4ff70c97c87a97bc7f000002"
			},
			{
				"_id" : ObjectId("4ff70c97c87a97bc7f000002")
			}
		]
	},
	"ntoreturn" : 1,
	"nscanned" : 1,
	"nreturned" : 1,
	"responseLength" : 657,
	"millis" : 0,
	"client" : "0.0.0.0",
	"user" : ""
}
{
	"ts" : ISODate("2012-08-15T02:51:43.241Z"),
	"op" : "update",
	"ns" : "gimmebar.assets",
	"query" : {
		"_id" : ObjectId("4ff70c97c87a97bc7f000002"),
		"$or" : [
			{
				"_id" : "4ff70c97c87a97bc7f000002"
			},
			{
				"_id" : ObjectId("4ff70c97c87a97bc7f000002")
			}
		]
	},
	"updateobj" : {
		"$set" : {
			"private" : true
		}
	},
	"nscanned" : 1,
	"fastmod" : true,
	"millis" : 0,
	"client" : "0.0.0.0",
	"user" : ""
}
{
	"ts" : ISODate("2012-08-15T02:51:43.241Z"),
	"op" : "query",
	"ns" : "gimmebar.assets",
	"query" : {
		"_id" : ObjectId("4ff70c97c87a97bc7f000002")
	},
	"ntoreturn" : 1,
	"idhack" : true,
	"responseLength" : 657,
	"millis" : 0,
	"client" : "0.0.0.0",
	"user" : ""
}
{
	"ts" : ISODate("2012-08-15T02:51:43.241Z"),
	"op" : "command",
	"ns" : "gimmebar.$cmd",
	"command" : {
		"findAndModify" : "assets",
		"query" : {
			"$or" : [
				{
					"_id" : "4ff70c97c87a97bc7f000002"
				},
				{
					"_id" : ObjectId("4ff70c97c87a97bc7f000002")
				}
			]
		},
		"update" : {
			"$set" : {
				"private" : true
			}
		},
		"new" : true,
		"upsert" : false
	},
	"ntoreturn" : 1,
	"responseLength" : 763,
	"millis" : 0,
	"client" : "127.0.0.1",
	"user" : ""
}
{
	"ts" : ISODate("2012-08-15T02:51:47.096Z"),
	"op" : "command",
	"ns" : "gimmebar.$cmd",
	"command" : {
		"profile" : 0
	},
	"ntoreturn" : 1,
	"responseLength" : 58,
	"millis" : 0,
	"client" : "127.0.0.1",
	"user" : ""
}
> 

Note on line 44 how it queries explicitly for the _id (assuming this is from the first query), and also uses the $or block. This doesn't seem right (and makes for strange explain() results (an empty BasicCursor for the $or clause).

Looks like the _id is appended to the query, explicitly, here, but that doesn't explain leaving the $or clause. https://github.com/mongodb/mongo/blob/master/src/mongo/db/commands/find_and_modify.cpp#L119