Unable to export the collection from firebase

Hi, I have a collection named Orders then in that collection document with orderid which contains collection of of LocationHistory i.e. Collection -> Document -> Collection. When i run export command

fei --serviceAccount firebase-adminsdk.json --ref "Orders" --file export.json --export --override

It generates following file

{ "metadata": {"timestamp":"2019-11-26_14-56-36","ref":"Orders"} }