How to delete a deep nested object in mongodb

Say I have a document that represents books like this:

{
   "_id":"1234567890",
   "title":"Lord Of The Rings",
   "books": {
      "1234567890":{
         "_id":"123456789890",
         "title":"The Two Towers",
         "page_count":{
            "en":6000,
            "de":7000
         }
      },
      "2234567890":{
         "_id":"223456789890",
         "title":"The Return Of The King",
         "page_count":{
            "en":6000,
            "de":7000
         }
      },
   }
}

How would I go about removing the page count for the 2nd book? I tried

{$unset : {"books.2234567890.page_count":""}}

Didn't work. Any ideas?

Thanks a lot