$select after API action returns empty body

We have defined an action on SOShipment and exposed it via the REST API.  When we call the action for the API we define a query string param $select=ShipmentNbr.  The action succeeds, but the the return body is empty.  We expected to see an SOShipment, with the ShipmentNbr specified.  How can we fix this?

Here are the details

On SOShipmentEntryExt, we define an action named AddShipmentNbr.

        public PXAction<SOShipment> AddShipmentNbr;
        [PXUIField(DisplayName = "Add ShipmentNbr",
            MapEnableRights = PXCacheRights.Select,
            MapViewRights = PXCacheRights.Select)]
        [PXButton()]
        public virtual IEnumerable addShipmentNbr(PXAdapter adapter)
        {
            ...
        }

We exposed this action as an Action on the Shipment REST API, naming it LockShipmentMutex.

We construct a call in Postman

POST {{PackingEndpointURL}}/Shipment/LockShipmentMutex?$select=ShipmentNbr

BODY

{
    "entity": {
        "id": "6B4E9A21-78F6-EA11-96A4-000D3A610F1B"
    }
}

The action succeeds, but we receive an empty body in the response.  We expected to receive a SOShipment entity with ShipmentNbr field specified.