Can't update records from Parabola - typecast doesn't work with PATCH?

Struggling with parabola ‘send to api’ module while trying to update an airtable table.
I’m doing an update (patch) and I’m trying to send some numerical values.

{
  "records": [
    {
      "id": "{inventory record id}",
 "fields": {
 "sku":"{sku}",
 "in stock":"{in stock}",
 "prev stock":"{prev stock}",
 "updated":"{updated}"
        }
    }
  ]
}

If I send this I get an error stating that the ‘in stock’ and ‘prev stock’ can’t accept the values (because airtable is seeing the values as text trying to be inserted into a number field).

This works fine when creating records (Post) as I use typecast: true. It doesn’t work for Patch though even if I add typecast: true. It throws a different error.

If I change the json to this and remove the quotes from the value fields :-

{
  "records": [
    {
      "id": "{inventory record id}",
 "fields": {
 "sku":"{sku}",
 "in stock":{in stock},
 "prev stock":{prev stock},
 "updated":{updated}
        }
    }
  ]
}

Then it says my json isn’t valid.

Any ideas?

Thanks

Jonathan