Trying to update a nested field of an object that already exists as a value type, will result in an error when executed from the mongo shell:

> doc =

{t1:null} { "t1" : null }

> db.test.insert(doc)
> db.test.find()

{ "_id" : ObjectId("529c58a9642f618ff689a5eb"), "t1" : null }

> db.test.update(

{ "_id" : ObjectId("529c58a9642f618ff689a5eb")}

,{"$set":{"t1.t2":1}})
LEFT_SUBFIELD only supports Object: t1 not: 10

Trying the same with the JAVA driver (which I access from MATLAB, but I suppose native JAVA would be similar):
>> doc(1) = BasicDBObject('t1','');
>> db.getCollection('test').insert(doc)
>> db.getCollection('test').update(BasicDBObject('_id',doc(1).get('_id')),BasicDBObject('$set',BasicDBObject('t1.t2',1)))

This fails but returns no error (also other valid fields in doc won't be updated)