java.lang.NullPointerException at org.mvel2.ast.IntSub.getReducedValueAccelerated

When I run my planning I get now and then the stacktrace as mentioned below.
It happens about 1 in 5 times during running a JUnit test on small amount of planning entities and problem facts.

It started after adding my last rule:

rule "tasksInSameCommunityMustBeChained"
when
$lead : Task(this==lead, period!=null, communityId!= null, $communityId : communityId)
not ( Task(this==lead, communityId == $communityId, id != $lead.id, ($lead.startPeriodId-1) <= endPeriodId && ($lead.endPeriodId+1) >= startPeriodId ) )
then
insertLogical(new IntConstraintOccurrence("tasksInSameCommunityMustBeChained", ConstraintType.NEGATIVE_HARD, Importance.chainedPeriod, $communityId, $lead, $lead.getStartPeriodId()));
end

Stack trace:

Exception in thread "Thread-1" java.lang.NullPointerException
at org.mvel2.ast.IntSub.getReducedValueAccelerated(IntSub.java:17)
at org.mvel2.compiler.ExecutableAccessor.getValue(ExecutableAccessor.java:38)
at org.mvel2.ast.Substatement.getReducedValueAccelerated(Substatement.java:44)
at org.mvel2.ast.BinaryOperation.getReducedValueAccelerated(BinaryOperation.java:108)
at org.mvel2.ast.And.getReducedValueAccelerated(And.java:34)
at org.mvel2.MVELRuntime.execute(MVELRuntime.java:85)
at org.mvel2.compiler.CompiledExpression.getDirectValue(CompiledExpression.java:123)
at org.mvel2.compiler.CompiledExpression.getValue(CompiledExpression.java:119)
at org.mvel2.compiler.CompiledExpression.getValue(CompiledExpression.java:113)
at org.mvel2.MVEL.executeExpression(MVEL.java:930)
at org.drools.rule.constraint.MvelConditionEvaluator.evaluate(MvelConditionEvaluator.java:70)
at org.drools.rule.constraint.MvelConditionEvaluator.ensureBranchEvaluation(MvelConditionEvaluator.java:117)
at org.drools.rule.constraint.MvelConditionEvaluator.ensureCompleteEvaluation(MvelConditionEvaluator.java:102)
at org.drools.rule.constraint.MvelConditionEvaluator.ensureCompleteEvaluation(MvelConditionEvaluator.java:90)
at org.drools.rule.constraint.MvelConditionEvaluator.getAnalyzedCondition(MvelConditionEvaluator.java:82)
at org.drools.rule.constraint.MvelConstraint.executeJitting(MvelConstraint.java:270)
at org.drools.rule.constraint.MvelConstraint.access$200(MvelConstraint.java:51)
at org.drools.rule.constraint.MvelConstraint$ConditionJitter.run(MvelConstraint.java:250)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)