MongoDB update document with _id query

I'm using nodejs and the native mongo driver to update document in my collection. I can't query for the _id in my update statement.

  db.users.update({
       _id: user._id
   }, {
       $set: {
           userName: user.userName,
           email: user.email,
           icon: user.icon,
           role: user.role
       }
   }, {
       multi: false
   }, next);

what is the right way of doing it?