I'm trying to use the enabled function to ensure that some rules only fire under certain conditions. I have the following in my DRL : rule "Load pricelist" salience 10000 when $unitPrice : Pricelist() from PricelistUtilities.getPriceLists() then insert ($unitPrice); end rule "Retract a priced unit" salience 5000 when $priceUnit : PriceUnit( basePrice != null ) then System.out.println("Retracting ..." + $priceUnit); retract ( $priceUnit ); end rule "Price Unit by Prodid, Grade, Pattern, Color" enabled ( $detailPrice == true ) salience 2500 when $priceUnit : PriceUnit ( $detailPrice : detailPrice ) $priceList : Pricelist( prodid == $priceUnit.prodid, coverGrade == $priceUnit.coverGrade, coverSeries == $priceUnit.coverSeries, coverColor == $priceUnit.coverColor ) then modify ( $priceUnit ) { setBasePrice ( $priceList.getBasePrice() ); } end rule "Price Unit by Prodid, Grade, Pattern" enabled ( $detailPrice == true ) salience 2000 when $priceUnit : PriceUnit ( $detailPrice : detailPrice ) $priceList : Pricelist( prodid == $priceUnit.prodid, coverGrade == $priceUnit.coverGrade , coverSeries == $priceUnit.coverSeries ) then modify ( $priceUnit ) { setBasePrice ( $priceList.getBasePrice() ); } end rule "Price Unit by Prodid, Grade, Pattern 0000" salience 1500 when $priceUnit : PriceUnit ( ) $priceList : Pricelist( prodid == $priceUnit.prodid, coverGrade == $priceUnit.coverGrade , coverSeries == "0000" ) then System.out.println("Pricing unit : " + $priceUnit + " from " + $priceList); $priceUnit.setBasePrice($priceList.getBasePrice()); update ( $priceUnit ); end Before adding the 'enabled' conditions to the rules, it all worked fine. But after adding the 'enabled' function, I'm getting the following NullPointer exception : Exception executing consequence for rule "Price Unit by Prodid, Grade, Pattern 0000" in com.sample: java.lang.NullPointerException at org.drools.runtime.rule.impl.DefaultConsequenceExceptionHandler.handleException(DefaultConsequenceExceptionHandler.java:39) at org.drools.common.DefaultAgenda.fireActivation(DefaultAgenda.java:1283) at org.drools.common.DefaultAgenda.fireNextItem(DefaultAgenda.java:1209) at org.drools.common.DefaultAgenda.fireAllRules(DefaultAgenda.java:1442) at org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:710) at org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:674) at org.drools.impl.StatefulKnowledgeSessionImpl.fireAllRules(StatefulKnowledgeSessionImpl.java:230) at com.generic.rules.utils.DroolsTest.main(DroolsTest.java:37) Caused by: java.lang.NullPointerException at org.drools.common.DefaultAgenda.createActivation(DefaultAgenda.java:569) at org.drools.reteoo.RuleTerminalNode.modifyLeftTuple(RuleTerminalNode.java:297) at org.drools.reteoo.SingleLeftTupleSinkAdapter.propagateModifyChildLeftTuple(SingleLeftTupleSinkAdapter.java:246) at org.drools.reteoo.JoinNode.modifyLeftTuple(JoinNode.java:363) at org.drools.reteoo.LeftTupleSource.doModifyLeftTuple(LeftTupleSource.java:297) at org.drools.reteoo.LeftTupleSource.modifyLeftTuple(LeftTupleSource.java:271) at org.drools.reteoo.JoinNode.modifyLeftTuple(JoinNode.java:429) at org.drools.reteoo.CompositeLeftTupleSinkAdapter.doPropagateModifyObject(CompositeLeftTupleSinkAdapter.java:260) at org.drools.reteoo.CompositeLeftTupleSinkAdapter.propagateModifyObject(CompositeLeftTupleSinkAdapter.java:275) at org.drools.reteoo.LeftInputAdapterNode.modifyObject(LeftInputAdapterNode.java:170) at org.drools.reteoo.CompositeObjectSinkAdapter.doPropagateModifyObject(CompositeObjectSinkAdapter.java:507) at org.drools.reteoo.CompositeObjectSinkAdapter.propagateModifyObject(CompositeObjectSinkAdapter.java:432) at org.drools.reteoo.ObjectTypeNode.modifyObject(ObjectTypeNode.java:314) at org.drools.reteoo.EntryPointNode.modifyObject(EntryPointNode.java:265) at org.drools.common.NamedEntryPoint.update(NamedEntryPoint.java:470) at org.drools.common.NamedEntryPoint.update(NamedEntryPoint.java:370) at org.drools.base.DefaultKnowledgeHelper.update(DefaultKnowledgeHelper.java:317) at org.drools.base.DefaultKnowledgeHelper.update(DefaultKnowledgeHelper.java:312) at com.sample.Rule_Price_Unit_by_Prodid__Grade__Pattern_0000_8fe2d406670448989b7e4f1e051a27e9.defaultConsequence(Rule_Price_Unit_by_Prodid__Grade__Pattern_0000_8fe2d406670448989b7e4f1e051a27e9.java:9) at com.sample.Rule_Price_Unit_by_Prodid__Grade__Pattern_0000_8fe2d406670448989b7e4f1e051a27e9DefaultConsequenceInvokerGenerated.evaluate(Unknown Source) at com.sample.Rule_Price_Unit_by_Prodid__Grade__Pattern_0000_8fe2d406670448989b7e4f1e051a27e9DefaultConsequenceInvoker.evaluate(Unknown Source) at org.drools.common.DefaultAgenda.fireActivation(DefaultAgenda.java:1273) ... 6 more In the console I also see the results of my System.out.println : Pricing unit : PriceUnit [prodid=001001, coverGrade= A, coverSeries=1234, coverColor=05, basePrice=null] from Pricelist [prodid=001001, cover= A0000null, basePrice=599] The only 'null' is $priceUnit.basePrice, which is what I'm trying to set. I only get the exception when the 'detailedPrice' of PriceUnit is set to false (i.e. when "Price Unit by Prodid, Grade, Pattern 0000" is attempting to fire. If I set detailedPrice to true, the other rules fire just fine. If I comment out the 'update ( $priceUnit );', the consequence fires just fine, but then other rules fire as well. When PriceUnit's pattern is set to anything other than '1234', then too consequence of ""Price Unit by Prodid, Grade, Pattern 0000"" rule fires just fine and I get the following output (correctly so) : Pricing unit : PriceUnit [prodid=001001, coverGrade= A, coverSeries=3456, coverColor=05, basePrice=null] from Pricelist [prodid=001001, cover= A0000null, basePrice=599] Retracting ...PriceUnit [prodid=001001, coverGrade= A, coverSeries=3456, coverColor=05, basePrice=599] What is causing the 'NullPointerException' ? I'm using Drools 5.4.0. Please help. 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