Google Checkout PHP response handler serial number not sent

[nbelgard]

This is probably a really easy question to answer, but I'll ask it anyway :3I've got my site set up using the Google Checkout PHP sample code (I'm working with the Sandbox at the moment). I have it so that I can add items to a shopping cart on my site (custom-built), and they'll be sent to Google using the PHP libraries. This all works fine, and I can pay using the sandbox account. However, when I receive a response back using the Notification API, I have an issue.I can receive new order notifications, and they work fine. However, when I send my acknowledgement of the notification (which I assume happens by way of \[code\]$Gresponse->SendAck();\[/code\]), I get an error in my Integration Console saying I've not sent the serial number with my request.My question is: Has anyone run into this problem before, and how do I solve it?I'm using the \[code\]responsehandlerdemo.php\[/code\] file in the sample code (I've tweaked it a little by adding PHP \[code\]mail()\[/code\] functions in to debug the order notifications), and I've got Google Checkout set up using the XML API - sending the notification as XML (yes, I'm using a HTTPS link).