Have a similar issue (same error) just using CoreData with a transformable attribute with a NSSecureUnarchiveFromDataTransformer transformer and a Custom Class of Set<String>. Frustratingly this error happens in some unit tests, but not consistently. Using NSSet appears to be working for the moment, but I'm not confident that it's gone unless I can reproduce the issue consistently.

Appreciate the answer above does appear to fix the issue, but this isn't really an ideal solution, and doesn't help us understand how we should be using transformers really. Specifically I feel like there's code smell around type erasure as a solution here, given that we're dealing with Foundation objects.

sorted now, thanks!