Mongodb.dropUser（）引用错误：未定义

我不明白？我已使用root用户登录

> use mydb
switched to db mydb
> show users
{
    "_id" : "mydb.mydbReadWrite",
    "user" : "mydbReadWrite",
    "db" : "mydb",
    "roles" : [
        {
            "role" : "readWrite",
            "db" : "mydb"
        }
    ]
}

> db.dropUser(mydbReadWrite)
2015-03-29T03:55:52.276+0200 E QUERY    ReferenceError: mydbReadWrite is not defined
    at (shell):1:13

db.dropUser(mydbReadWrite)

而是使用：

db.dropUser("mydbReadWrite")