why the code passes Servlet container test but fails in StrutsTestCase

[????]

Hi, 

I was trying to use StrutsTestCase to write a test for a Bean, which has a 
member of String[]. The code runs perfectly in servlet container(Tomcat), but 
fails as a mock test. In EditActionTest.java, assertNotNull("The action is null 
but should not be.", action) still works fine, but String result = 
actionProxy.execute() returns the result as "error" instead of "success". All 
the related codes are listed below.

Can anyone give me a hint?
Thanks in advance! Tom 

Person.java
public class Person{
   String[] models = new String[]{"Ford", "Nissan"};

   public void setModels(String[] models){
      this.models = models;
   }

   public String[] getModels(){
      return models;
   }
}

Edit.java
public class EditAction extends ActionSupport{
   Person personBean;

   public void execute(){      
      return SUCCESS;
   }
}

struts.xml
<struts>
      <action name="edit" class="EditAction" method="execute">
     <result name="input">/edit.jsp</result>
         <result name="success">/thankyou.jsp</result>
      </action>
</struts>

edit.jsp
<%@ taglib prefix="s" uri="/struts-tags" %>
<s:form action="save" method="post">
<s:checkboxlist key="personBean.models" list="Ford, Nissan" />
<s:submit key="submit" />
</s:form>

EditActionTest.java
public class EditActionTest extends StrutsTestCase {
   @Test 
   public void testExecuteValidationPasses() throws Exception {
      request.setParameter("personBean.models", new String[]{"Ford", "Nissan"});
      ActionProxy actionProxy = getActionProxy("/edit.action") ;      
      EditAction action = (EditAction) actionProxy.getAction();
      assertNotNull("The action is null but should not be.", action);  
      String result = actionProxy.execute();
      assertEquals("The execute method did not return " + ActionSupport.SUCCESS 
+ " but should have.", ActionSupport.SUCCESS, result);
   }
}