mongodb sub document trouble

gistfile1.js

db.foo.insert( {_id: 1}, { "$set": {param: "one" }} )

db.foo.update(
    {
  "_id": 1, 
	"sub._id": 2
    },
    { $set: { "sub.$": {
	"param": 2,
	"place": "Boston"
    } } },
    {"upsert":true}
)
// returns: E11000 duplicate key error index: test.foo.$_id_  dup key: { : 1.0 }