findAndModify is confusing me

gistfile1.txt

> db.messages.find()
{ "_id" : "msgid:test:e9f1aca0-2d86-11e0-9695-138376569236", "status" : "incomplete", "expires" : 1296598229, "timeout" : 1296512429 }
{ "_id" : "msgid:test:f93bf066-2d87-11e0-ab0b-d7db09943f10", "status" : "incomplete", "expires" : 1296598684, "timeout" : 1296512884 }

> db.messages.find( { _id : "msgid:test:f93bf066-2d87-11e0-ab0b-d7db09943f10" } )
{ "_id" : "msgid:test:f93bf066-2d87-11e0-ab0b-d7db09943f10", "status" : "incomplete", "expires" : 1296598684, "timeout" : 1296512884 }

> cmd = {
...   findAndModify : "messages",
...   query  : { _id :  "msgid:test:f93bf066-2d87-11e0-ab0b-d7db09943f10" },
...   update : { $inc : { attempts : 1 } },
...   new    : true,
...   fields : { "_id" : 0, "attempts" : 1 }
... };
{
        "findAndModify" : "messages",
        "query" : {
                "_id" : "msgid:test:f93bf066-2d87-11e0-ab0b-d7db09943f10"
        },
        "update" : {
                "$inc" : {
                        "attempts" : 1
                }
        },
        "new" : true,
        "fields" : {
                "_id" : 0,
                "attempts" : 1
        }
}
>
> db.runCommand(cmd);
{ "errmsg" : "No matching object found", "ok" : 0 }
>