putListingsItem example, to create/update offer for an ASIN

I am trying to use the putListingsItem operation which was added by @altruer in #64.

const res = await sellingPartner.callAPI({
  operation: "putListingsItem",
  endpoint: "listingsItems",
  path: {
    sellerId: process.env.AMAZON_SELLER_ID,
    sku: PRODUCT_SKU
  },
  query: {
    marketplaceIds: [MARKETPLACE_ID],
  },
  body: {
    productType: "COMPUTER_DRIVE_OR_STORAGE",
    requirements: "LISTING_OFFER_ONLY",
    attributes: {
      "condition_type": [{
        "value": "new_new",
        "marketplace_id": MARKETPLACE_ID
      }],
      "external_product_id": [{
        "value": "B07FP5L3ZQ", // < This is the ASIN
        "marketplace_id": MARKETPLACE_ID
      }],
      "external_product_type": [{
        "value": "ASIN",
        "marketplace_id": MARKETPLACE_ID
      }],
    }
  }
})

The error I am getting is:

...
{ message: 'The attributes are invalid.',
severity: 'ERROR',
attributeName: 'item_sku' }
...

Any help on this would be highly appreciated.