When extending from an @NaturalIdCache annotated entity then following exception raises during MetadataBuilding:

java.lang.ClassCastException: org.hibernate.mapping.SingleTableSubclass cannot be cast to org.hibernate.mapping.RootClass
at org.hibernate.boot.internal.InFlightMetadataCollectorImpl.addEntityBinding(InFlightMetadataCollectorImpl.java:312)
at org.hibernate.cfg.AnnotationBinder.bindClass(AnnotationBinder.java:808)
at org.hibernate.boot.model.source.internal.annotations.AnnotationMetadataSourceProcessorImpl.processEntityHierarchies(AnnotationMetadataSourceProcessorImpl.java:250)
at org.hibernate.boot.model.process.spi.MetadataBuildingProcess$1.processEntityHierarchies(MetadataBuildingProcess.java:231)
at org.hibernate.boot.model.process.spi.MetadataBuildingProcess.complete(MetadataBuildingProcess.java:274)
at org.hibernate.boot.model.process.spi.MetadataBuildingProcess.build(MetadataBuildingProcess.java:84)
at org.hibernate.boot.internal.MetadataBuilderImpl.build(MetadataBuilderImpl.java:474)
at org.hibernate.boot.internal.MetadataBuilderImpl.build(MetadataBuilderImpl.java:1)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:689)
...

Pull request inlcuding fix and testcase will follow very soon..