"Failed to convert item" when saving a Visitor Group.

I'm implementing a Visitor Group Criterion which allows the user to select a category. I've borrowed the code from the built-in Visited Category Criterion but when I save the Visior Group I get an error saying  "Failed to convert item". I'm debugging my solution when I do this and there's no sign of an exception being throwin in the Output window for me to latch onto. I'm at a loss.

I've found this blog post but it lacks any real explanation for the issue:

https://world.episerver.com/forum/legacy-forums/Episerver-7-CMS/Thread-Container/2012/12/VisitorGroupCriterion-displays-Failed-to-convert-item/?pageIndex=2

What's causing the "Failed to convert item" error?

I'm using latest EPiServer.