Zoho Books: Automation - Custom Function - Item is not updating

I am trying to update category of the item with custom function. Below is the custom function. Category is a custom field I need to update.

itemID = item.get("item_id");

organizationID = organization.get("organization_id");

jsonstring = "{\"Category\":\"Composites\"}";

// To update the current record

result = zoho.books.updateRecord("Item",organizationID,itemID,jsonstring.toMap());

It is not updating the category value for the item. No error generated also.