I accidently denoted the wrong class with the annotator @PlanningEntityCollectionProperty ; a class which was NOT a planning entity. Apparently, Drools Planner does not attempt to check for such a possibility, hence I get: *--OUTPUT--* Exception in thread "main" java.lang.NullPointerException at org.drools.planner.core.domain.solution.SolutionDescriptor.getAllFacts(SolutionDescriptor.java:135) at org.drools.planner.core.solution.director.DefaultSolutionDirector.getWorkingFacts(DefaultSolutionDirector.java:129) at org.drools.planner.core.solution.director.DefaultSolutionDirector.resetWorkingMemory(DefaultSolutionDirector.java:123) at org.drools.planner.core.solution.director.DefaultSolutionDirector.setWorkingSolution(DefaultSolutionDirector.java:97) at org.drools.planner.core.solver.DefaultSolver.setPlanningProblem(DefaultSolver.java:97) at in.co.technovia.examduties.ExamDutiesApp.main(ExamDutiesApp.java:47) *--OUTPUT--* Please check for such a possibility in future versions of Drools Planner and emit a more meaningful error message. -- View this message in context: http://drools.46999.n3.nabble.com/Trivial-bug-in-Planner-5-4-Beta1-mislea... Sent from the Drools: User forum mailing list archive at Nabble.com.