"The invoice has been created. We face an error while sending the invoice. Reason: Email subject cannot be blank."

"The invoice has been created. We face an error while sending the invoice. Reason: Email subject cannot be blank."

Hi,

I am trying to send an invoice via

https://invoice.zoho.com/api/v3/invoices?send=true&authtoken=xxxxxxxxxx (xxxxxxxxxx is my authtoken)

with some JSONString.

IN response, I get the invoiceId as :4373120000000*****

When I GET query:
https://invoice.zoho.com/api/v3/invoices/4373120000000*****/email?authtoken=***********

The response I get has a field:

"subject": "Invoice - INV-000008 from RecurPay",

which means the subject is clearly set.

Why doesnt the email get sent then?