NullPointerException at org.drools.core.common.LeftTupleSetsImpl.removeInsert(LeftTupleSetsImpl.java:141)

Exception in thread "main" java.lang.NullPointerException
	at org.drools.core.common.LeftTupleSetsImpl.removeInsert(LeftTupleSetsImpl.java:141)
	at org.drools.core.common.LeftTupleSetsImpl.addDelete(LeftTupleSetsImpl.java:80)
	at org.drools.core.phreak.PhreakAccumulateNode.doLeftDeletes(PhreakAccumulateNode.java:603)
	at org.drools.core.phreak.PhreakAccumulateNode.doNode(PhreakAccumulateNode.java:65)
	at org.drools.core.phreak.RuleNetworkEvaluator.switchOnDoBetaNode(RuleNetworkEvaluator.java:565)
	at org.drools.core.phreak.RuleNetworkEvaluator.evalBetaNode(RuleNetworkEvaluator.java:536)
	at org.drools.core.phreak.RuleNetworkEvaluator.evalNode(RuleNetworkEvaluator.java:372)
	at org.drools.core.phreak.RuleNetworkEvaluator.innerEval(RuleNetworkEvaluator.java:332)
	at org.drools.core.phreak.RuleNetworkEvaluator.outerEval(RuleNetworkEvaluator.java:166)
	at org.drools.core.phreak.RuleNetworkEvaluator.evaluateNetwork(RuleNetworkEvaluator.java:123)
	at org.drools.core.phreak.RuleExecutor.reEvaluateNetwork(RuleExecutor.java:194)
	at org.drools.core.phreak.RuleExecutor.fire(RuleExecutor.java:142)
	at org.drools.core.phreak.RuleExecutor.evaluateNetworkAndFire(RuleExecutor.java:74)
	at org.drools.core.common.DefaultAgenda.fireNextItem(DefaultAgenda.java:978)
	at org.drools.core.common.DefaultAgenda.fireAllRules(DefaultAgenda.java:1292)
	at org.drools.core.impl.StatefulKnowledgeSessionImpl.internalFireAllRules(StatefulKnowledgeSessionImpl.java:1294)
	at org.drools.core.impl.StatefulKnowledgeSessionImpl.fireAllRules(StatefulKnowledgeSessionImpl.java:1281)
	at org.drools.core.impl.StatefulKnowledgeSessionImpl.fireAllRules(StatefulKnowledgeSessionImpl.java:1260)
	at org.drools.core.command.runtime.rule.FireAllRulesCommand.execute(FireAllRulesCommand.java:109)
	at org.drools.core.command.runtime.rule.FireAllRulesCommand.execute(FireAllRulesCommand.java:34)
	at org.drools.core.impl.StatelessKnowledgeSessionImpl.execute(StatelessKnowledgeSessionImpl.java:292)
	at Test.main(Test.java:13)

rule "test"
when
    not( String() )
    accumulate(
        Long();
        sum(1)
    )
    ( Boolean() or not( Float() ) )
then
    insert(new String());
end

<?xml version="1.0" encoding="UTF-8"?>
<kmodule xmlns="http://jboss.org/kie/6.0.0/kmodule" />

import org.kie.api.KieBase;
import org.kie.api.KieServices;
import org.kie.api.runtime.StatelessKieSession;
import org.kie.internal.command.CommandFactory;

public class Test {

    public static void main(String[] args) {

        KieBase kieBase = KieServices.Factory.get().getKieClasspathContainer()
                .getKieBase();
        StatelessKieSession session = kieBase.newStatelessKieSession();
        session.execute(CommandFactory.newFireAllRules());
    }
}