Error Missing or empty field exchange thrown on placeOrder()

When i place order (using the PHP api placeOrder(), i am getting the below error:

Fatal error: Uncaught InputException (400) 'Missing or empty field `exchange`' thrown in kiteconnect.php on line 899

I understood the API placeOrder() already uses Content-type: application/x-www-form-urlencoded and i don't need to set it explicitly.

Please help.

Thanks