Books API Update an Recurring Invoice -> Error 11

Books API Update an Recurring Invoice -> Error 11

Hello,

i have several Recurring Invoices in my Zoho Books.
Now i must update them via The Zoho Creator. The Update work over an selvemade PHP CURL Script that is explained here.

The Auth Token and Org ID are correct, but if i make a PUT over Postman (because my script give an Error) i become the (same) error:

{
    "code": 11,
    "message": "The parameter JSONString is mandatory."
}

But the Body field is filled with the Correct Required Data from Here:
https://www.zoho.com/books/api/v3/#Recurring-Invoices_Update_Recurring_Invoice


Body Text and Result Via Postman:



Headers Data:



Can Anyone tell me what i am doing wrong?! Is there any other field required? Or is the API Buggy?

Thanks a lot.


Kind Regards,
Klaus