Delivery extension sample fails with 'Invalid Class Name'


I posted this on Stack Overflow first since it might have a better chance of being responded to there.

I've done nothing more than compile the sample SSRS delivery extension and deploy it to the server as per instructions. When I make a call to the ReportService2010 ListExtensions web service method, the extension shows up in the list of results. However, when I go to create a subscription in the Report Manager, as soon as I choose the Printer Delivery Sample from the drop-down list of available delivery extensions, I get this error:
Invalid Class Name
I'm running SSRS 2012 SP2 (11.0.5569.0) on Windows Server 2012 R2.