求助：使用SimpleTagSupport自定义标签JspFragment空指针异常

我的标签类如下：
Java code:public class ForCyc extends SimpleTagSupport{
private int index;
private int sum;
public void setIndex(int index) {
this.index = index;
}
public void setSum(int sum) {
this.sum = sum;
}
 public void doTag() throws JspException, IOException {
    JspFragment jspFragment = this.getJspBody();  //jspFragment 为空
    JspWriter out = getJspContext().getOut();        
    StringWriter writer = new StringWriter();
    jspFragment.invoke(writer);
    String str = writer.toString();       
    out.println(str);
}
}tld code:  <tag>
   <name>for</name>
   <tag-class>servlet.ForCyc</tag-class>
   <body-content>scriptless</body-content>
   <variable>
   <name-given>frag</name-given>
   </variable>
   <attribute>
   <name>index</name>  
   <required>true</required>
        <rtexprvalue>true</rtexprvalue>
   <type>int</type>
   </attribute>
   <attribute>
   <name>sum</name>  
   <required>true</required>
        <rtexprvalue>true</rtexprvalue>
   <type>int</type>
   </attribute>
  </tag>JSP:    <c-1_0-rt:set var="test"/>
<for:for index="1" sum="10">
${test}
test
<c-1_0-rt:out value="test"></c-1_0-rt:out>
</for:for>谢谢各位了~~！