Mongoose searching on array subdocument ObjectId not working

This is honestly driving me crazy. Here's the problem:

I'm running the following query in Mongoose:

s.findSubdocument=function(uid, cb)
{
     this.model('User').find({"flwng._id":uid.toString()}).select('flwng').exec(cb);
}

on the following User Schema:

var userSchema= new mongoose.Schema(
    {
        uname:{type:String, index:{sparse:true, unique:true, dropDups:true}}, //the username
        email:String,
        pwd:{type:String},
        flwng:[{_id : {type : mongoose.Schema.ObjectId},uname : String}], //_id of users I am following
        flwrsz:Number,
        flwngsz:Number,
        feedsz:Number,
    }, {j: 1});//j:1 means guarantee it is written to the journal.

userSchema.set('autoIndex', true);
userSchema.index({"fid":1}, {sparse:true, unique:true, dropDups:true});
userSchema.index({"flwng._id" : 1});

Where uid="53c4f16c431247694f0000a3" ==> but I get an empty array back :(

When I run the same exact query in the mongo shell:

db.users.find({"flwng._id":"53c4f16c431247694f0000a3"});

I get the right set of results back. I tried with and without an index and schema on "flwng._id", I tried to drop the index, reIndex and I'm now running out of ideas. Am I doing something wrong with Mongoose?

Any help would be appreciated - thanks! Henri