diff --git a/ganttzk/src/main/java/org/zkoss/ganttz/data/resourceload/LoadPeriod.java b/ganttzk/src/main/java/org/zkoss/ganttz/data/resourceload/LoadPeriod.java
index cbd639834..0784dd709 100644
--- a/ganttzk/src/main/java/org/zkoss/ganttz/data/resourceload/LoadPeriod.java
+++ b/ganttzk/src/main/java/org/zkoss/ganttz/data/resourceload/LoadPeriod.java
@@ -82,6 +82,8 @@ public class LoadPeriod {
             public int compare(LoadPeriod o1, LoadPeriod o2) {
                 if (o1.overlaps(o2)) {
                     LOG.warn(o1 + " overlaps with " + o2);
+                    throw new IllegalArgumentException(o1 + " overlaps with "
+                            + o2);
                 }
                 int comparison = compareLocalDates(o1.start, o2.start);
                 if (comparison != 0)