Using Set in accumulate causes NPE in some scenarios

When using either "collectSet" or the full syntax (init, action, reverse, result) of "accumulate" an NPE is beeing thrown in org.drools.rule.Accumulate.reverse(final Object[] workingMemoryContext, final Object[] context, ...). Actual exception is comming from a generated accumulator within the reverse() method of Accumulate.

IMPORTANT: When using Rule 2 (full syntax) I was able to replicate that the NPE is thrown during the execution of "reverse" block of "accumulate"!
IMPORTANT: I checked trivial stuff such as fields/variables/collections used in rules not null and some other sanity checks.

I am attaching

• example rules causing NPE;
• Stack trace;
• test project as soon as I have set it up;

• Rule 1 causing NPE

  rule "Sub optimal resource parallelism"
      when
          $project : Project($optimalResourceParallelism : optimalResourceParallelism)
          $assignment : Assignment(project == $project, $leftId : id, resource != null, $leftResource : resource)
          exists Assignment(project == $project, id != $leftId, resource != null && != $leftResource)
          $resourceParallelism : Set ((size + 1) != $optimalResourceParallelism)
          	from accumulate ( $otherAssignment : Assignment(project == $project, id != $leftId, resource != null && != $leftResource, $resource : resource),
          	collectSet($resource) )
      then
      	insertLogical(new IntConstraintOccurrence("sub-optimal resource parallelism", ConstraintType.NEGATIVE_SOFT
      	, Math.abs($optimalResourceParallelism - ($resourceParallelism.size() + 1)), $assignment));
          System.out.println("Sub optimal resource parallelism detected. Assignment: " + $assignment.getId()
          + " optimalParallelism is: " + $optimalResourceParallelism + " and actual: " + ($resourceParallelism.size() + 1));
  end

• Rule 2 causing NPE

  rule "Sub optimal resource parallelism"
      when
          $project : Project($optimalResourceParallelism : optimalResourceParallelism)
          $assignment : Assignment(project == $project, $leftId : id, resource != null, $leftResource : resource)
          exists Assignment(project == $project, id != $leftId, resource != null && != $leftResource)
          $resourceParallelism : Set ((size + 1) != $optimalResourceParallelism)
          	from accumulate ( $otherAssignment : Assignment(project == $project, id != $leftId, resource != null && != $leftResource, $resource : resource),
          	init( Set<Resource> resources = new HashSet<Resource>(); ),
          	action( resources.add($resource); ),
          	reverse( resources.remove($resource); ),
          	result( resources.size(); ) )
      then
      	insertLogical(new IntConstraintOccurrence("sub-optimal resource parallelism", ConstraintType.NEGATIVE_SOFT
      	, Math.abs($optimalResourceParallelism - ($resourceParallelism.size() + 1)), $assignment));
          System.out.println("Sub optimal resource parallelism detected. Assignment: " + $assignment.getId()
          + " optimalParallelism is: " + $optimalResourceParallelism + " and actual: " + ($resourceParallelism.size() + 1));
  end

• Stack trace

  org.drools.RuntimeDroolsException: java.lang.NullPointerException
  	at org.drools.rule.Accumulate.reverse(Accumulate.java:217)
  	at org.drools.reteoo.AccumulateNode.removeMatch(AccumulateNode.java:888)
  	at org.drools.reteoo.AccumulateNode.modifyRightTuple(AccumulateNode.java:554)
  	at org.drools.reteoo.BetaNode.modifyObject(BetaNode.java:431)
  	at org.drools.reteoo.CompositeObjectSinkAdapter.doPropagateModifyObject(CompositeObjectSinkAdapter.java:468)
  	at org.drools.reteoo.CompositeObjectSinkAdapter.propagateModifyObject(CompositeObjectSinkAdapter.java:436)
  	at org.drools.reteoo.AlphaNode.modifyObject(AlphaNode.java:149)
  	at org.drools.reteoo.CompositeObjectSinkAdapter.doPropagateModifyObject(CompositeObjectSinkAdapter.java:468)
  	at org.drools.reteoo.CompositeObjectSinkAdapter.propagateModifyObject(CompositeObjectSinkAdapter.java:436)
  	at org.drools.reteoo.ObjectTypeNode.modifyObject(ObjectTypeNode.java:284)
  	at org.drools.reteoo.EntryPointNode.modifyObject(EntryPointNode.java:271)
  	at org.drools.common.NamedEntryPoint.update(NamedEntryPoint.java:465)
  	at org.drools.common.AbstractWorkingMemory.update(AbstractWorkingMemory.java:959)
  	at org.drools.common.AbstractWorkingMemory.update(AbstractWorkingMemory.java:932)
  	at org.drools.planner.core.heuristic.selector.variable.PlanningValueWalker.changeWorkingValue(PlanningValueWalker.java:125)
  	at org.drools.planner.core.heuristic.selector.variable.PlanningValueWalker.walk(PlanningValueWalker.java:112)
  	at org.drools.planner.core.heuristic.selector.variable.PlanningVariableWalker.walk(PlanningVariableWalker.java:130)
  	at org.drools.planner.core.constructionheuristic.greedyFit.decider.DefaultGreedyDecider.decideNextStep(DefaultGreedyDecider.java:62)
  	at org.drools.planner.core.constructionheuristic.greedyFit.DefaultGreedyFitSolverPhase.solve(DefaultGreedyFitSolverPhase.java:62)
  	at org.drools.planner.core.solver.DefaultSolver.runSolverPhases(DefaultSolver.java:166)
  	at org.drools.planner.core.solver.DefaultSolver.solve(DefaultSolver.java:138)
  	at de.orbitx.retena.domain.ScheduleTest.schedule(ScheduleTest.java:227)
  	at de.orbitx.retena.domain.ScheduleTest.testGetScoreTrivialScenario(ScheduleTest.java:78)
  	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:45)
  	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
  	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:42)
  	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
  	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:263)
  	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:68)
  	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:47)
  	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:231)
  	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:60)
  	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:229)
  	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:50)
  	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:222)
  	at org.junit.runners.ParentRunner.run(ParentRunner.java:300)
  	at org.apache.maven.surefire.junit4.JUnit4TestSet.execute(JUnit4TestSet.java:35)
  	at org.apache.maven.surefire.junit4.JUnit4Provider.executeTestSet(JUnit4Provider.java:115)
  	at org.apache.maven.surefire.junit4.JUnit4Provider.invoke(JUnit4Provider.java:97)
  	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
  	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
  	at java.lang.reflect.Method.invoke(Method.java:597)
  	at org.apache.maven.surefire.booter.ProviderFactory$ClassLoaderProxy.invoke(ProviderFactory.java:103)
  	at $Proxy0.invoke(Unknown Source)
  	at org.apache.maven.surefire.booter.SurefireStarter.invokeProvider(SurefireStarter.java:150)
  	at org.apache.maven.surefire.booter.SurefireStarter.runSuitesInProcess(SurefireStarter.java:91)
  	at org.apache.maven.surefire.booter.ForkedBooter.main(ForkedBooter.java:69)
  Caused by: java.lang.NullPointerException
  	at org.drools.base.accumulators.CollectSetAccumulateFunction.reverse(CollectSetAccumulateFunction.java:123)
  	at org.drools.base.accumulators.JavaAccumulatorFunctionExecutor.reverse(JavaAccumulatorFunctionExecutor.java:130)
  	at org.drools.rule.Accumulate.reverse(Accumulate.java:208)
  	... 51 more