Due date of invoice not updating after API Call

I am trying to update the due date on an invoice via an API Call but it seems to be not working.

I am using the yyyy-MM-dd format

json.put("due_date",newDueDate);

json.put("due_date_formatted",newDueDate);

json.put("reason","Updated Amount Calculation");

customeUpdateMapString = json.toString();

//info customeUpdateMapString;

urlToUse = "https://books.zoho.com/api/v3/invoices/" + invoiceID + "?organization_id=" + organizationID;

updateInvoice = invokeurl

[

url :urlToUse

type :PUT

parameters:customeUpdateMapString

connection:"books_connection"

];

Any idea what I am missing? Ive tried using Custom under payment terms in the api call but that causes an error.