API call to create create invoice in Zoho Inventory

I am trying to send the following json to create an invoice:
{
"customer_id": 1167451000000062085, // found by calling the get contact request
"invoice_number": "INV-00002",
"date": "2018-02-04",
"due_date": "2018-02-10",
"line_items": [
{
"item_id": 1167451000000062103, // found by calling the get items request
"quantity": 130
}
]
}

but the response is:
{
"code": 4,
"message": "Invalid value passed for JSONString"
}

the post request is being sent to :
https://inventory.zoho.com/api/v1/invoices?authtoken=<token>&organization_id=<org_id>
and content-type: application/json.
Is the request be of type "text" with the "JSONString" appended to the body? Any help will be highly beneficial to me.

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