ZohoBooks API to get the contents of an Invoice

ZohoBooks API to get the contents of an Invoice

As defined 
https://www.zoho.com/books/api/v3/#Invoices_Get_an_invoice

I am able to get the invoice via Curl

curl https://books.zoho.com/api/v3/invoices?organization_id=YYYYYYYYY\&invoice_number=INV-XXXXX
-H 'Authorization: Zoho-oauthtoken ZZZZZZZZZZZZZZZ'

ALL INVOICE DETAILS ARE RETURNED AS EXPECTED

Performing the following POST in postman

https://books.zoho.com/api/v3/invoices?organization_id=YYYYYYYYYYYY&invoice_number=INV-XXXX
With a header set to
Authorization: Zoho-oauthtoken ZZZZZZZZZZ

Returns back with
{
    "code"4,
    "message""Invalid value passed for JSONString"
}

Since there is no JSON information needed to be passed (per the API Manual)
What am i missing in the Postman example?