Hi. I am experiencing the following exception: org.drools.RuntimeDroolsException: Exception executing ReturnValue constraint org.drools.rule.ReturnValueRestriction@a5f6c234 : java.lang.NullPointerException. The rule that is causing the exception to occur is the following: rule "spreadOvertime" when $violatingTask : MaintenanceTask(eval(maintenanceSlot.getAvailability() == 1), motor != null, maintenanceTechnician != null, $overtimeTaskID : id); ArrayList( $numberOfViolations : size ) from collect( MaintenanceTask( id == ($overtimeTaskID + 1) || id == ($overtimeTaskID + 2) || id == ($overtimeTaskID - 1) || id == ($overtimeTaskID - 2), eval(maintenanceSlot.getAvailability() >= 1) ) ); then insertLogical(new IntConstraintOccurrence("spreadOvertime", ConstraintType.NEGATIVE_SOFT, $numberOfViolations, $violatingTask, $numberOfViolations)); //System.out.println("Rule Fired: spreadOvertime ( number of violations:" + $numberOfViolations + " for task " + $violatingTask + ")"); end The purpose of the rule is to spread the closer to normal maintenance shifts. Just to clarrify what the objects are: MaintenanceTask: Consists of a MaintenanceSlot, MaintenanceTechnician and Motor MaintenanceSlot: Day, Timeslot, Availability and Index The 'availability' of a MaintenanceSlot specifies whether it is considered Overtime, ProductionTime or Normal Maintenance This exception is thrown at the end of solving as the solver (I presume) is supposed to return the best solution. The score is calculating and updating but at the end of the specified step count, this exception occurs. If I comment this rule out, no exception. Any help would be appreciated! Regards, De Ville -- View this message in context: http://drools-java-rules-engine.46999.n3.nabble.com/RuntimeDroolsExceptio... Sent from the Drools - User mailing list archive at Nabble.com.