The RTE is caused by following DRL.g in writing a function; it says you can omit the type for a parameter. A function like this causes the crash: function inf sqr( a ){ return a*a; } Untyped parameters may be used in queries. I suppose it will be necessary to use different NTs for parameters and param_definition. Exception in thread "main" java.lang.RuntimeException: org.drools.compiler.DroolsParserException: Unknown error while parsing. This is a bug. Please contact the Development team. at org.drools.compiler.PackageBuilder.addKnowledgeResource(PackageBuilder.java:545) at org.drools.builder.impl.KnowledgeBuilderImpl.add(KnowledgeBuilderImpl.java:28) at rss.drools.setup.Application.setup(Application.java:40) at rss.drools.setup.Application.execute(Application.java:60) at appl.transport.Transport.main(Transport.java:64) Caused by: org.drools.compiler.DroolsParserException: Unknown error while parsing. This is a bug. Please contact the Development team. at org.drools.compiler.DrlParser.compile(DrlParser.java:250) at org.drools.compiler.DrlParser.parse(DrlParser.java:142) at org.drools.compiler.DrlParser.parse(DrlParser.java:147) at org.drools.compiler.PackageBuilder.addPackageFromDrl(PackageBuilder.java:264) at org.drools.compiler.PackageBuilder.addKnowledgeResource(PackageBuilder.java:459)