Index: src/test/java/org/quartz/CronTriggerTest.java
===================================================================
--- src/test/java/org/quartz/CronTriggerTest.java	(revision 0)
+++ src/test/java/org/quartz/CronTriggerTest.java	(revision 0)
@@ -0,0 +1,47 @@
+/* 
+ * Copyright 2007 OpenSymphony 
+ * 
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not 
+ * use this file except in compliance with the License. You may obtain a copy 
+ * of the License at 
+ * 
+ *   http://www.apache.org/licenses/LICENSE-2.0 
+ *   
+ * Unless required by applicable law or agreed to in writing, software 
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT 
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the 
+ * License for the specific language governing permissions and limitations 
+ * under the License.
+ */
+package org.quartz;
+
+import junit.framework.TestCase;
+
+import java.text.ParseException;
+
+/**
+ * Unit test for CronTrigger.
+ */
+public class CronTriggerTest extends TestCase {
+    
+    public void testClone() throws ParseException {
+        CronTrigger trigger = new CronTrigger("test", "testgroup", "0 0 12 * * ?");
+        CronTrigger trigger2 = (CronTrigger) trigger.clone();
+
+        assertEquals( "Cloning failed", trigger, trigger2 );
+
+        // equals() doesn't test the cron expression
+        assertEquals( "Cloning failed for the cron expression", 
+                      "0 0 12 * * ?", trigger2.getCronExpression()
+                    );
+    }
+
+    // http://jira.opensymphony.com/browse/QUARTZ-558
+    public void testQuartz558() throws ParseException {
+        CronTrigger trigger = new CronTrigger("test", "testgroup");
+        CronTrigger trigger2 = (CronTrigger) trigger.clone();
+
+        assertEquals( "Cloning failed", trigger, trigger2 );
+    }
+
+}
Index: src/java/org/quartz/CronTrigger.java
===================================================================
--- src/java/org/quartz/CronTrigger.java	(revision 656)
+++ src/java/org/quartz/CronTrigger.java	(working copy)
@@ -437,7 +437,9 @@
     
     public Object clone() {
         CronTrigger copy = (CronTrigger) super.clone();
-        copy.setCronExpression((CronExpression)cronEx.clone());
+        if (cronEx != null) {
+            copy.setCronExpression((CronExpression)cronEx.clone());
+        }
         return copy;
     }