Integration Issue Detail

I got this error:

XML GC Received:
<?xml version="1.0" encoding="UTF-8"?>
<add-merchant-order-number xmlns="http://checkout.google.com/schema/2" google-order-number="">
<merchant-order-number></merchant-order-number>
</add-merchant-order-number>
--------------------------------------------------------------------------
XML GC Sent:
<?xml version="1.0" encoding="UTF-8"?>
<error xmlns="http://checkout.google.com/schema/2" serial-number="86fce692-202b-460a-87bf-77a5f3d649">
<error-message>Ids are integral numbers, but you sent: </error-message>
</error>


Any Ideas why and how to fix this?