Sorting treetable column generated using columngenerator

Hi,

Please check the code below

TreeTable tt = new TreeTable();
tt.addContainerProperty("check", String.class, null);
for (int i = 1; i < 6; i++) {
  String parent = "parent - " + i;
  tt.addItem(parent);
  for (int j = 1; j < 3; j++) {
     String child = "child - " + i + j;
     tt.addItem(child);
     tt.setParent(child, parent);
   }
}

tt.addGeneratedColumn("check", new ColumnGenerator() {

   @Override
   public Object generateCell(Table source, Object itemId, Object columnId) {
       Label label = new Label();
       label.setValue(itemId.toString());
       return label;
   }
 });

In the UI, clicking on the header doesnot sort the coIumn.
Please check attachment.

Thanks in advance