I resorted to rolling back our Drools version to 5.0.1 and the code works without changes. So...guess this must be a bug with 5.1.0-M1. -- View this message in context: http://drools-java-rules-engine.46999.n3.nabble.com/NullPointerException-modify-tp818403p824720.html Sent from the Drools - User mailing list archive at Nabble.com.