Realm java freeze not freezing the object

Using below code:

TxnModel oldTxnModel = uiThreadRealm.where(TxnModel.class).equalTo("_id", new ObjectId(_id)).findFirst();

oldTxnModel.freeze();
Log.e(TAG, "isFrozen() : " + oldTxnModel.isFrozen());
//isFrozen() : false

Here isFrozen() should be true or I’m doing something wrong?