matches operator on variable cause intermittent thread fork and throws exception

The java code that does this multiple insertion and firing is like this: Main.java package drools; import org.drools.KnowledgeBase; import org.drools.KnowledgeBaseFactory; import org.drools.builder.KnowledgeBuilder; import org.drools.builder.KnowledgeBuilderFactory; import org.drools.builder.ResourceType; import org.drools.definition.KnowledgePackage; import org.drools.event.rule.DebugWorkingMemoryEventListener; import org.drools.io.ResourceFactory; import org.drools.runtime.StatefulKnowledgeSession; import java.util.Collection; public class Main { static StatefulKnowledgeSession ksession; public static void main( String [] argv) throws Exception { initRules(); // fire once to run the no condition LHS ksession.fireAllRules(); for ( int i=0;i < 100; i++) { Person person = new Person(); person.setName( "Test" ); ksession.insert(person); ksession.fireAllRules(); } } private static void initRules() { KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder(); // this will parse and compile in one step kbuilder.add(ResourceFactory.newClassPathResource( "main.drl" , Main.class), ResourceType.DRL); // Check the builder for errors if (kbuilder.hasErrors()) { System .out.println(kbuilder.getErrors().toString()); throw new RuntimeException( "Unable to compile \" main.drl\ "." ); } // get the compiled packages (which are serializable) Collection<KnowledgePackage> pkgs = kbuilder .getKnowledgePackages(); // add the packages to a knowledgebase (deploy the knowledge packages). KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase(); kbase.addKnowledgePackages(pkgs); ksession = kbase.newStatefulKnowledgeSession(); ksession.addEventListener( new DebugWorkingMemoryEventListener()); } } The Person is a simple POJO Person.java public class Person { private String name; public String getName() { return name; } public void setName( String name) { this .name = name; } } The rule file that does the matching is like this: main.drl package drools; dialect "mvel" declare Stuff regexp : String end rule "Test Regex" salience 100 when then Stuff s = new Stuff( "Test" ) System .out.println( "Insert reg ex variable" ); insert (s); end rule "Test Equality" salience 10 when Stuff( $regexp : regexp ) Person( name matches $regexp ) //Person( name matches "Test" ) then System .out.println( "Found name" ); end rule "Dump person" when $p : Person( ) then retract ( $p ) end