findAndModify regression 2.0.x -> v2.2.0rc0 with positional updates

> db.foo.find()

{ "_id" : ObjectId("5010009b54f3e488821117fc"), "j" : 0, "s" : "abc", "e" : [ { "t" : "a", "c" : 100 } ] }

> db.runCommand( { findAndModify: "foo", query : { s : "abc", j : 0 , 'e.t' : "a" }, update : { $set : { 'e.$.c' : 3 } }  });

{

        "value" : {

                "_id" : ObjectId("5010009b54f3e488821117fc"),

                "j" : 0,

                "s" : "abc",

                "e" : [

                        {

                                "t" : "a",

                                "c" : 100

                        }

                ]

        },

        "errmsg" : "exception: can't append to array using string field name [$]",

        "code" : 13048,

        "ok" : 0

}