Edson, Thanks for the tip. The "not Building()" works just fine, so long as I don't assert the null building anyway. I guess I was going for pure abstraction of all the rules, including null even though deep down I knew it didn't make sense to assert a null. The workaround is to hard code the null case like this: private void validateGetBuilding( Building building ) throws BOException { // set up an exception boException = new BOException(); // send the exception into the BRE FactHandle boFact = businessRuleSession.insert( boException ); // send the building into the BRE FactHandle buildingFact = null; if( building != null ) { buildingFact = businessRuleSession.insert( building ); } // trigger the validate building agenda items businessRuleSession.setFocus("validate delete or get building"); // fire businessRuleSession.fireAllRules(); // retract items businessRuleSession.retract(boFact); if( buildingFact != null ) { businessRuleSession.retract(buildingFact); } // check on the exception (is set in the rule) if( boException.getCode() != null ) { log.debug("Exception for building=" + building); String message = BOMessages.getProperty( boException.getCode() ); boException.setMessage(message); throw boException; } } With the rule in question being reconfigured to this: rule "building is null" agenda-group "validate delete or get building" salience 10 when boException : BOException() not Building() then boException.setCode("BO-0008"); end This ends up throwing the expected BOException: 08:16:26,093 DEBUG BuildingBO:288 = Exception for building=null 08:16:26,119 ERROR InCastAPI:73 = BO-0008: The building is null Don't you just love the green bar in junit? Thanks for you help -- really enjoying developing this app with Rules. On 8/14/07, Edson Tirelli <tirelli(a)post.com&gt; wrote: -- Thanks, Stuart Stuart Moffatt Disaster Resistant University University of Utah (801) 585-9122 stuart.moffatt(a)utah.edu