Creating a shipment via REST fails when including SShipUnit

Summary:

I am trying to add a shipment to OTM for rating. The shipment was executed in another system and needs to be imported to OTM solely for rating and financial processes. I am trying to create the shipment including its SShipUnit and corresponding Lines.

Content (please ensure you mask any confidential information):

When posting the contents to logisticsRestApi/resources-int/v2/shipments I get the following error:

{
  "status": 400,
  "title": "Content error in message body.",
  "detail": "Invalid attribute sShipUnits for resource shipmentNoSSUs."
}

However, the REST documentation states:

A shipment ship unit can only be created in two cases. One, when it is provided as a part of the shipment payload.