NotEval incurs NPE with boolean column

tajo> select * from bool_test where col1 is false;
java.lang.NullPointerException
        at org.apache.tajo.engine.eval.NotEval.postOrder(NotEval.java:91)
        at org.apache.tajo.engine.eval.EvalTreeUtil.findDistinctRefColumns(EvalTreeUtil.java:74)
        at org.apache.tajo.engine.planner.PlannerUtil.canBeEvaluated(PlannerUtil.java:307)
        at org.apache.tajo.engine.planner.rewrite.FilterPushDownRule.visitScan(FilterPushDownRule.java:238)
        at org.apache.tajo.engine.planner.rewrite.FilterPushDownRule.visitScan(FilterPushDownRule.java:38)
        at org.apache.tajo.engine.planner.BasicLogicalPlanVisitor.visitChild(BasicLogicalPlanVisitor.java:93)