Actually the spec says that Bean#getStereotypes() must only return valid stereotypes. Which means that it should complain if a class which is not a Stereotype gets returned. Otoh null is a perfectly valid value and shall just get handled the same like an empty Set for that matter.

This happens in InvalidStereotypeExtension as detected by Reinhard Sandtner. Maybe you wanted to return PlainOldAnnotation.class instead of null?