NullpointerException in Accumulate when use query

FACT: Person
package test;

public class Person {

private String firstname;
private String secondname;
private String address;

public Person() {
}

public String getFirstname()

{ return firstname; }

public void setFirstname(String firstname)

{ this.firstname = firstname; }

public String getSecondname()

{ return secondname; }

public void setSecondname(String secondname)

{ this.secondname = secondname; }

public String getAddress()

{ return address; }

public void setAddress(String address)

{ this.address = address; }

}

r1.drl:
package test1
import test.Person
import java.util.HashSet

query find(String $first, String $second)
p: Person(firstname == "111", secondname==$second)
end

rule "test"
no-loop true
when
$list: HashSet( size >= 0 )
from accumulate (
Person($addr: address, firstname=="Tom", secondname=="Cat"),
init ( HashSet set = new HashSet(); ),
action ( set.add($addr); ),
reverse ( set.remove($addr); ),
result(set) )
then
System.out.println("It' OK~!");

end

Test.java
package test;

import java.util.Collection;

import org.drools.KnowledgeBase;
import org.drools.KnowledgeBaseFactory;
import org.drools.builder.KnowledgeBuilder;
import org.drools.builder.KnowledgeBuilderError;
import org.drools.builder.KnowledgeBuilderErrors;
import org.drools.builder.KnowledgeBuilderFactory;
import org.drools.builder.ResourceType;
import org.drools.definition.KnowledgePackage;
import org.drools.io.ResourceFactory;
import org.drools.runtime.StatefulKnowledgeSession;
import org.drools.runtime.rule.FactHandle;

public class Test {
public static void main(String[] args) throws Exception

{ new Test().test(); }

public void test() throws Exception {
KnowledgeBase base = KnowledgeBaseFactory.newKnowledgeBase();
StatefulKnowledgeSession s = base.newStatefulKnowledgeSession();
new Th(s).start();
Thread.sleep(2000);

KnowledgeBuilder builder = KnowledgeBuilderFactory.newKnowledgeBuilder();
builder.add(ResourceFactory.newClassPathResource("r1.drl", getClass()), ResourceType.DRL);
KnowledgeBuilderErrors errs = builder.getErrors();
if (!errs.isEmpty()) {
for (KnowledgeBuilderError e: errs)

{ System.out.println(e.toString()); }

}
Collection<KnowledgePackage> col = builder.getKnowledgePackages();
base.addKnowledgePackages(col);
}

class Th extends Thread {
StatefulKnowledgeSession s;
public Th(StatefulKnowledgeSession s)

{ this.s = s; }

public void run() {
Person p = new Person();
p.setFirstname("Tom");
p.setSecondname("Cat");
p.setAddress("Mars");
FactHandle fact = s.insert(p);
int i=0;
while (true) {
s.update(fact, p);
s.fireAllRules();
try

{ Thread.sleep(1000); }

catch (InterruptedException e) {}
i++;
}
}
}
}

Run Test.java, will throws Exception:

Exception in thread "Thread-0" org.drools.RuntimeDroolsException: java.lang.NullPointerException
at org.drools.rule.Accumulate.reverse(Accumulate.java:212)
at org.drools.reteoo.AccumulateNode.removeMatch(AccumulateNode.java:902)
at org.drools.reteoo.AccumulateNode.modifyRightTuple(AccumulateNode.java:537)
at org.drools.reteoo.BetaNode.modifyObject(BetaNode.java:531)
at org.drools.reteoo.SingleObjectSinkAdapter.propagateModifyObject(SingleObjectSinkAdapter.java:68)
at org.drools.reteoo.AlphaNode.modifyObject(AlphaNode.java:157)
at org.drools.reteoo.SingleObjectSinkAdapter.propagateModifyObject(SingleObjectSinkAdapter.java:68)
at org.drools.reteoo.AlphaNode.modifyObject(AlphaNode.java:157)
at org.drools.reteoo.CompositeObjectSinkAdapter.doPropagateModifyObject(CompositeObjectSinkAdapter.java:507)
at org.drools.reteoo.CompositeObjectSinkAdapter.propagateModifyObject(CompositeObjectSinkAdapter.java:421)
at org.drools.reteoo.ObjectTypeNode.modifyObject(ObjectTypeNode.java:314)
at org.drools.reteoo.EntryPointNode.modifyObject(EntryPointNode.java:265)
at org.drools.common.NamedEntryPoint.update(NamedEntryPoint.java:470)
at org.drools.common.AbstractWorkingMemory.update(AbstractWorkingMemory.java:960)
at org.drools.common.AbstractWorkingMemory.update(AbstractWorkingMemory.java:933)
at org.drools.impl.StatefulKnowledgeSessionImpl.update(StatefulKnowledgeSessionImpl.java:284)
at test.Test$Th.run(Test.java:53)
Caused by: java.lang.NullPointerException
at test1.Rule_test_b59970efe0844a8d8797758b863454b1$Accumulate0.reverse(Rule_test_b59970efe0844a8d8797758b863454b1.java:52)
at test1.Rule_test_b59970efe0844a8d8797758b863454b1Accumulate0Invoker.reverse(Rule_test_b59970efe0844a8d8797758b863454b1Accumulate0Invoker.java:69)
at org.drools.rule.Accumulate.reverse(Accumulate.java:203)
... 16 more

But, if I change the query to like this:
query find(String $first, String $second)
p: Person(firstname == "111", secondname==$second)
end
It will be ok.

However if I change the query to like this:
query find(String $first, String $second)
p: Person(firstname == "Tom", secondname==$second)
end
It will throws Exception again.
firstname=="Tom" is a condition of rule "test"