orderBefore causes exception

package com.mom.jcrperf.tests;

import javax.jcr.Node;
import javax.jcr.NodeIterator;
import javax.jcr.RepositoryException;
import javax.jcr.Session;

/**
 * Measure the speed of reordering child nodes.
 * 
 * Create 10 parents Give each one 100 children Reorder the children in reverse
 * order
 */
public class ReorderChildNodesTest extends PerformanceTest {

     private static final int PARENT_COUNT = 10;
     private static final int CHILD_COUNT = 100;

     private Session session;

     private Node node;

     private String bigProperty = "this is a nice big string";

     public void beforeSuite() throws RepositoryException {

          for (int i = 0; i < 8; i++) {
               bigProperty = bigProperty + bigProperty + i;
          }

          session = getRepository().login(getCredentials());
          node = session.getRootNode().addNode("testnode", "nt:unstructured");
          for (int i = 0; i < PARENT_COUNT; i++) {

               Node parent = node.addNode("node" + i, "nt:unstructured");
               parent.addMixin("mix:versionable");
               parent.setProperty("prop_" + i, bigProperty + i);
               
               session.save();
               
               parent.checkout();

               for (int j = 0; j < CHILD_COUNT; j++) {
                    Node child = parent.addNode("Child " + j, "nt:unstructured");
                    child.addMixin("mix:versionable");
                    child.setProperty("prop_" + j, bigProperty + j);
               }
               parent.save();
               parent.checkin();
          }
          session.save();
     }

     public void beforeTest() throws RepositoryException {
          //
     }

     public void runTest() throws Exception {

          for (NodeIterator i = node.getNodes(); i.hasNext();) {

               Node parent = i.nextNode();
               parent.checkout();
               
               int kid = 0;

               for (NodeIterator j = parent.getNodes(); j.hasNext();) {
                    Node child = j.nextNode();
                    parent.orderBefore("Child " + kid, "Child 0");
                    kid++;
               }
               parent.save(); // !!!! Moving this code up into the loop eliminates the problem !!!!
               parent.checkin();
          }
          session.save();
     }

     public void afterTest() throws RepositoryException {
          session.save();
     }

     public void afterSuite() throws RepositoryException {
          session.getRootNode().getNode("testnode").remove();
          session.save();
          session.logout();
     }

}