CNFE thrown, no reasonable error message due to global type using type arguments

1. no imports here!
   global Set<String> nameSet;

causes:
java.lang.ClassNotFoundException: Unable to find class 'Set<String>'
at org.drools.base.ClassTypeResolver.resolveType(ClassTypeResolver.java:215)
at org.drools.compiler.PackageBuilder.mergePackage(PackageBuilder.java:946)
at org.drools.compiler.PackageBuilder.newPackage(PackageBuilder.java:920)
at org.drools.compiler.PackageBuilder.addPackage(PackageBuilder.java:631)
at org.drools.compiler.PackageBuilder.addPackageFromDrl(PackageBuilder.java:310)
at org.drools.compiler.PackageBuilder.addKnowledgeResource(PackageBuilder.java:486)
at org.drools.builder.impl.KnowledgeBuilderImpl.add(KnowledgeBuilderImpl.java:28)

kBuilder.getErrors() returns an irregular error message (no line number, etc.)
GlobalError: nameSet

Expected: no useless stack dump, proper error message.