diff --git a/navalplanner-gantt-zk/src/main/java/org/zkoss/ganttz/data/GanttDiagramGraph.java b/navalplanner-gantt-zk/src/main/java/org/zkoss/ganttz/data/GanttDiagramGraph.java
index 6500b974b..1ae2607a1 100644
--- a/navalplanner-gantt-zk/src/main/java/org/zkoss/ganttz/data/GanttDiagramGraph.java
+++ b/navalplanner-gantt-zk/src/main/java/org/zkoss/ganttz/data/GanttDiagramGraph.java
@@ -163,6 +163,11 @@ public class GanttDiagramGraph {
         rulesEnforcersByTask.remove(task);
         topLevelTasks.remove(task);
         update(outgoing);
+        if (task.isContainer()) {
+            for (Task t : task.getTasks()) {
+                remove(t);
+            }
+        }
     }
 
     private void updateOutgoing(Task task) {