NPE when trying to add the same resource twice

 
        String str =
                "rule R when\n" +
                "  $s : String()\n" +
                "then\n" +
                "end\n";

        KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();
        kbuilder.add( ResourceFactory.newByteArrayResource( str.getBytes() ), ResourceType.DRL );
        kbuilder.add( ResourceFactory.newByteArrayResource( str.getBytes() ), ResourceType.DRL );

java.lang.NullPointerException
	at org.drools.compiler.compiler.PackageBuilder.validateUniqueRuleNames(PackageBuilder.java:1351)
	at org.drools.compiler.compiler.PackageBuilder.initPackageRegistry(PackageBuilder.java:947)
	at org.drools.compiler.compiler.PackageBuilder.addPackage(PackageBuilder.java:899)
	at org.drools.compiler.compiler.PackageBuilder.addPackageFromDrl(PackageBuilder.java:484)
	at org.drools.compiler.compiler.PackageBuilder.addKnowledgeResource(PackageBuilder.java:688)
	at org.drools.compiler.builder.impl.KnowledgeBuilderImpl.add(KnowledgeBuilderImpl.java:51)
	at org.drools.compiler.builder.impl.KnowledgeBuilderImpl.add(KnowledgeBuilderImpl.java:40)