After upgrading from 2.4 to 2.6.0 our find_and_modify commands which use sorting started failing with the above error.

PyMongo generates the following command for find_and_modify with sorting:

{ findandmodify: "test", query: { _lock: { $lte: 9999999999999 } }, update: { $set: { _lock: 1 } }, sort: [ [ "_lock", 1 ] ] }

I tested it with 2.4 and it works fine, but on 2.6.0 it fails with the error (with the latest PyMongo 2.7):

> db.runCommand({ findandmodify: "test", query: { _lock: { $lte: 9999999999999 } }, update: { }, sort: [ [ "_lock", 1 ] ] })
{
	"errmsg" : "exception: nextSafe(): { $err: \"Can't canonicalize query: BadValue bad sort specification\", code: 17287 }",
	"code" : 13106,
	"ok" : 0
}

Looks like MongoDB has stopped respecting "sort: [ [ "_lock", 1 ] ]" syntax.

Is this a bug or expected change? This effectively breaks PyMongo's find_and_modify with sort functionality.