[rules-users] KnowledgeBuilderFactory.newKnowledgeBuilder(KnowledgeBase kbase) throws exception

I had some issues getting my codebase to work in Drools 5.6.0 (it worked in 5.5) so I am now trying the legacy api with 6.0.1Final. When I call: KnowledgeBuilderFactory.newKnowledgeBuilder(KnowledgeBase kbase) I get an exception: java.lang.ClassCastException: org.drools.impl.adapters.KnowledgeBaseAdapter cannot be cast to org.drools.core.impl.KnowledgeBaseImpl