Unfortunately not at the moment, as I've gone on holiday, however if you have access to the xtrareports....
* I created a very basic report, linked to a simple xpcollection.
* I added a grouping level and a field from the bound xpcollection to the grouping footer.
* Added an event to the summary calculation event (can't remember the exact event name)
* In the event code I had some buggy code which generated a null exception error - there wasn't anything fancy going on - you could just declare a null string and query it's properties to get the same effect.