worldpay futurepay cancellation - callback error

I receive the following error in my email when a customer cancels their futurepay with worldpay (test mode):
subject: Callback Failure Alert from WorldPay

Our systems have detected that your callback has failed.

This callback failure means we were unable to pass information
to your server about the following transaction:

Transaction ID:
Cart ID:
Installation ID: 1111111
Error reported: Unknown Select Message Type CAUSED BY A non-empty path is required

The callback is working well for the initial purchase using
http://<wpdisplay item=MC_callback>

I get the following error when I try to cancel a test purchase
Transaction declined (C)

Any ideas?