[Fulfillment Inbound API v2024-03-20] createInboundPlan "Prep classification" error

[MojoJake]

When trying to create an inbound plan it works and I get a planId and operationId. However when querying the status (using getInboundOperationStatus) so that I can proceed, I get the following error for for every book in the shipment.

"ERROR: Prep classification for this SKU was missing. Choose the prep category that applies to this SKU and apply any required prep and labeling to each sellable unit."

I'm not sure what a "Prep category" is, but I am providing the prepOwner and labelOwner fields as required in the docs.

Here is an example, using node, of trying to create a one book shipment.

let createRequest = {
    "contactInformation": {...},
    "destinationMarketplaces": ["ATVPDKIKX0DER"],
        "items" : [{
            "labelOwner": "SELLER",
            "prepOwner": "SELLER",
            "msku": "1234567",
            "quantity": 1
        }],
    "name": "TryingToShip101",
    "sourceAddress" : {...}
};
let createResponse = await spClient.callAPI({
    api_path: "/inbound/fba/2024-03-20/inboundPlans",
    method: "POST",
    options: {
        version: "2024-03-20"
    },
    body: createRequest
});

This create request works, but is there something missing from the single item in the "items" array of the request that needs to be specified? (note that changing from "SELLER" to "AMAZON" makes no difference)

Anyone else seeing this or know how to fix it?