cant append for subdocument level 2

It seems that i can go further than one subdocument if i want to add it dynamicaly, here is the code:

db.users.update(

{"pup.cmn.id":id}

, {"$addToSet":{"pup.cmn":

{"abus":email}

}})
this give error:

OperationFailure: can't append to array using string field name: cmn
then, if i add positional element i get this:

db.users.update(

{"pup.cmn.id":id}

, {"$addToSet":{"pup.$.cmn":

{"abus":email}

}})

"cmn" :
[
{
"fto" : ObjectId("5190e8a53a5f3a0c102af045")
"id" : "14.05.2013 12:29:53"
},
{
"abus" : "u...@example.com"
}
]
so as you can see, it will add it in the same level, and i dont want that, because the application will get errors.