Perhaps this has already been detected and fixed. 5.5.0 is still the last "Final", isn't it? // === express/Message.java package express; public class Message { private String text; public Message( String m ){ this.text = m; } public String getText(){return text; } } // === express/express.drl package express; declare CountedMessage extends Message count : int end // === Used with a straightforward build, this is sufficient to throw: Exception in thread "main" java.lang.NullPointerException at org.drools.base.ClassTypeResolver.resolveType(ClassTypeResolver.java:130) at org.drools.compiler.PackageBuilder.isNovelClass(PackageBuilder.java:2291) at org.drools.compiler.PackageBuilder.generateDeclaredBean(PackageBuilder.java:2473) at org.drools.compiler.PackageBuilder.processTypeDeclarations(PackageBuilder.java:2133) at org.drools.compiler.PackageBuilder.mergePackage(PackageBuilder.java:1272) at org.drools.compiler.PackageBuilder.addPackage(PackageBuilder.java:869) at org.drools.compiler.PackageBuilder.addPackageFromDrl(PackageBuilder.java:466) at org.drools.compiler.PackageBuilder.addKnowledgeResource(PackageBuilder.java:694) at org.drools.builder.impl.KnowledgeBuilderImpl.add(KnowledgeBuilderImpl.java:51) at org.drools.builder.impl.KnowledgeBuilderImpl.add(KnowledgeBuilderImpl.java:40) at express.Main.setup(Main.java:58) at express.Main.main(Main.java:118)