I get the following error in the Google Checkout log:

Ids are integral numbers, but you sent

<?xml version="1.0" encoding="UTF-8"?>
<send-buyer-message xmlns="http://checkout.google.com/schema/2" google-order-number="">
<message>tghg</message>
<send-email>true</send-email>
</send-buyer-message>

This issue is obvious... the order number from the cart is not being populated. Can someone tell how toget the order number added to the xml?