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