[Mongoose][Node.js] Cannot find document after insert

I'm dealing with a specific and simple case. I need some help, because I don't seem to be able to find the solution for this.

I've created a simple Schema and Model for MongoDB inside a javascript class.

this.UserSchema = new this.Schema({
    UserId: { type: String }, 
    IsToggle: { type: Boolean}
});

this.CalendarViewUserSchema = new this.Schema({ 
    _id:  { type: this.Schema.ObjectId },
    OwnerId: { type: String },
    UserList: [ this.UserSchema ] 
});

this.CalendarViewUser = this.mongoose.model('CalendarViewUsers',this.CalendarViewUserSchema);

This is in the constructor and I already have a lot of database interactions working, but I can't manage to get this one working.

I call this method for adding the new document:

AddCalendarViewUser : function(Id, UserToAddId){        
        NewUser = { UserId : UserToAddId, IsToggle : false };

        this.CalendarViewUser.update(
        { OwnerId : Id },
        { 
                       $set:  { OwnerId  : Id      },
           $push: { UserList : NewUser } 
                    },
        {upsert:true},
        function(err){
            if(err) {console.log(err);}
            else {console.log('Success');}
        }
    );
}, // this comma just introduces the next method.

This works fine. I can see the document is created correctly ,with Mongo Explorer, and if I use a .find() to retrieve everything from the collection, it shows up.

Then, I try to look for the inserted Doc with:

FindCalendarViewUser : function(Id){
    this.CalendarViewUser.findOne(
                { OwnerID : Id }, 
                function(err,result){

            if(err) {console.log(err);}
            else{console.log(result);}  
        }
        );
}

Here, it returns result = null.

The "Id" parameter is a String in both methods, just like the "OwnerId" in the Schema.

What am I doing wrong?

Both "OwnerId" and "Id" are the same. I also tried converting both to ObjectId, with no success.

Sorry for the code formatting.

EDIT:

Basically, I can find the Document using the _id field:

{ _id : '4fb0d1702edfda0104fc2b9f'}

But I can't find it with the OwnerId field:

{ OwnerId : '4f55e1a6c2c7287814000001'}

These values were taken directly from the database, so they correspond to the actual stored values.

EDIT 2:

I just figured out that if I insert the value by hand:

$set:  { OwnerId  : '4f55e1a6c2c7287814000001' }

and then search by the exact same string, it returns the document!

this.CalendarViewUser.findOne(
                    { OwnerID : '4f55e1a6c2c7287814000001'}
...

So I guess it is inserting some kind of garbage in the document's OwnerID field, or there was some kind of incompatibility with the field's Type. I will post the results.