I have encountered the following issue when running a rule that has more than 1 collect from a same collection. I have the following two rules: rule "filter sentences based on existence of attributes" when IPhoneQueryCmd(State.POPULATE == state, $tidt: targetId) $attributeFilteredSentences: ArrayList() from collect (IPhoneSentences.Sentence( ( (descriptor.targetPrivatename == false) || (descriptor.targetPrivatename == $cmt.sentenceFilters.targetPrivatename) ) && ( (descriptor.targetSurename == false) || (descriptor.targetSurename == $cmt.sentenceFilters.targetSurename) ) && ( (descriptor.targetFullname == false) || (descriptor.targetFullname == $cmt.sentenceFilters.targetFullname) ) && ( (descriptor.requesterPrivatename == false) || (descriptor.requesterPrivatename == $cmt.sentenceFilters.requesterPrivatename) ) && ( (descriptor.requesterSurename == false) || (descriptor.requesterSurename == $cmt.sentenceFilters.requesterSurenamename) ) && ( (descriptor.requesterFullname == false) || (descriptor.requesterFullname == $cmt.sentenceFilters.requesterFullname) ) && ( (descriptor.time == false) || (descriptor.time == $cmt.sentenceFilters.time) ) && ( (descriptor.subject == false) || (descriptor.subject == $cmt.subject) ) )) then end rule "Iphone targets information update - QUERY_AVAILABILITY" extends "filter sentences based on existence of attributes" when $qc: IPhoneQueryCmd(State.POPULATE == state, $tid: targetId) $cm: ConditionMatrix($taskId: taskId, targetId == $tid, contextIntention == EContextIntention.QUERY_AVAILABILITY, $timeConstraintType: requesterConstraint.constraintRequestType) $targetResponse: ArrayList() from collect (IPhoneSentences.Sentence(descriptor.intentions.intention contains EContextIntention.QUERY_AVAILABILITY.name) from $attributeFilteredSentences) then //do something End This works fine but once I wish to add a second filtered ArrayList using $attributeFilteredSentences I get a null pointer exception. For example I will add an exact similar filter condition but change the result var name (I called it $dummyVar) rule "Iphone targets information update - QUERY_AVAILABILITY" extends "filter sentences based on existence of attributes" when $qc: IPhoneQueryCmd(State.POPULATE == state, $tid: targetId) $cm: ConditionMatrix($taskId: taskId, targetId == $tid, contextIntention == EContextIntention.QUERY_AVAILABILITY, $timeConstraintType: requesterConstraint.constraintRequestType) $targetResponse: ArrayList() from collect (IPhoneSentences.Sentence(descriptor.intentions.intention contains EContextIntention.QUERY_AVAILABILITY.name) from $attributeFilteredSentences) $dummyVar: ArrayList() from collect (IPhoneSentences.Sentence(descriptor.intentions.intention contains EContextIntention.QUERY_AVAILABILITY.name) from $attributeFilteredSentences) then //do something End In this case I get the following exception stack: org.drools.runtime.rule.ConsequenceException: rule: Iphone dialog sentence - QUERY_AVAILABILITY at org.drools.runtime.rule.impl.DefaultConsequenceExceptionHandler.handleExcept ion(DefaultConsequenceExceptionHandler.java:39) at org.drools.common.DefaultAgenda.fireActivation(DefaultAgenda.java:927) at org.drools.common.DefaultAgenda.fireNextItem(DefaultAgenda.java:856) at org.drools.common.DefaultAgenda.fireAllRules(DefaultAgenda.java:1071) at org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.j ava:785) at org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.j ava:751) at org.drools.impl.StatefulKnowledgeSessionImpl.fireAllRules(StatefulKnowledgeS essionImpl.java:218) at org.drools.impl.StatelessKnowledgeSessionImpl.execute(StatelessKnowledgeSess ionImpl.java:302) at com.delegatecom.task.TaskStatelessSession.testRunRules(TaskStatelessSession. java:122) at com.delegatecom.task.TaskStatelessSession.internalTestNextStep(TaskStateless Session.java:98) at com.delegatecom.task.TaskStatelessSession.testNextStep(TaskStatelessSession. java:76) at com.delegatecom.test.AtoB.testStatelessRule(AtoB.java:99) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39 ) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl .java:25) at java.lang.reflect.Method.invoke(Method.java:597) at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod. java:44) at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.j ava:15) at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.ja va:41) at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.jav a:20) at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.jav a:76) at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.jav a:50) at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193) at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52) at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191) at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42) at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184) at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28 ) at org.junit.runners.ParentRunner.run(ParentRunner.java:236) at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestRef erence.java:46) at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:3 8) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRu nner.java:467) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRu nner.java:683) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner. java:390) at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner .java:197) Caused by: java.lang.NullPointerException at org.drools.reteoo.AccumulateNode.getFirstMatch(AccumulateNode.java:967) at org.drools.reteoo.AccumulateNode.splitList(AccumulateNode.java:920) at org.drools.reteoo.AccumulateNode.removePreviousMatchesForLeftTuple(Accumulat eNode.java:874) at org.drools.reteoo.AccumulateNode.retractLeftTuple(AccumulateNode.java:205) at org.drools.reteoo.CompositeLeftTupleSinkAdapter.doPropagateRetractLeftTuple( CompositeLeftTupleSinkAdapter.java:238) at org.drools.reteoo.CompositeLeftTupleSinkAdapter.propagateRetractLeftTupleDes troyRightTuple(CompositeLeftTupleSinkAdapter.java:138) at org.drools.reteoo.AccumulateNode.retractLeftTuple(AccumulateNode.java:212) at org.drools.reteoo.CompositeLeftTupleSinkAdapter.doPropagateRetractLeftTuple( CompositeLeftTupleSinkAdapter.java:238) at org.drools.reteoo.CompositeLeftTupleSinkAdapter.propagateRetractLeftTuple(Co mpositeLeftTupleSinkAdapter.java:122) at org.drools.reteoo.JoinNode.retractLeftTuple(JoinNode.java:163) at org.drools.reteoo.SingleLeftTupleSinkAdapter.doPropagateRetractLeftTuple(Sin gleLeftTupleSinkAdapter.java:217) at org.drools.reteoo.SingleLeftTupleSinkAdapter.propagateRetractLeftTuple(Singl eLeftTupleSinkAdapter.java:91) at org.drools.reteoo.JoinNode.retractLeftTuple(JoinNode.java:163) at org.drools.reteoo.SingleLeftTupleSinkAdapter.doPropagateRetractLeftTuple(Sin gleLeftTupleSinkAdapter.java:217) at org.drools.reteoo.SingleLeftTupleSinkAdapter.propagateRetractLeftTupleDestro yRightTuple(SingleLeftTupleSinkAdapter.java:107) at org.drools.reteoo.AccumulateNode.retractLeftTuple(AccumulateNode.java:212) at org.drools.reteoo.CompositeLeftTupleSinkAdapter.doPropagateRetractLeftTuple( CompositeLeftTupleSinkAdapter.java:238) at org.drools.reteoo.CompositeLeftTupleSinkAdapter.propagateRetractLeftTuple(Co mpositeLeftTupleSinkAdapter.java:122) at org.drools.reteoo.JoinNode.retractLeftTuple(JoinNode.java:163) at org.drools.reteoo.ModifyPreviousTuples.retractTuples(ModifyPreviousTuples.ja va:111) at org.drools.reteoo.EntryPointNode.modifyObject(EntryPointNode.java:176) at org.drools.common.AbstractWorkingMemory.update(AbstractWorkingMemory.java:14 42) at org.drools.common.AbstractWorkingMemory.update(AbstractWorkingMemory.java:13 49) at org.drools.base.DefaultKnowledgeHelper.update(DefaultKnowledgeHelper.java:18 3) at org.drools.base.DefaultKnowledgeHelper.update(DefaultKnowledgeHelper.java:19 6) at com.delegatecom.rules.drl.Rule_Iphone_dialog_sentence___QUERY_AVAILABILITY_0 .defaultConsequence(Rule_Iphone_dialog_sentence___QUERY_AVAILABILITY_0.java: 27) at com.delegatecom.rules.drl.Rule_Iphone_dialog_sentence___QUERY_AVAILABILITY_0 DefaultConsequenceInvoker.evaluate(Rule_Iphone_dialog_sentence___QUERY_AVAIL ABILITY_0DefaultConsequenceInvoker.java:65) at org.drools.common.DefaultAgenda.fireActivation(DefaultAgenda.java:917) ... 33 more Any idea what this can be? Regards, Hezi