帮忙看看!为什么checkbox为null呢?

<input type="checkbox" id="isBuy" name="isBuy" value="<%=news.getNewsID() %>"/>
<a href="dodeleteNews2.jsp">删除</a><!--删除页面-->
      request.setCharacterEncoding("GBK");
int count=0;
String[] checkbox=request.getParameterValues("isBuy");
NewsBiz newsBiz=new NewsBiz();
if(checkbox!=null && checkbox.length>0){ 
for(int i=0;i<checkbox.length;i++){
String strid= checkbox[i]; 
int id = Integer.parseInt(strid);
boolean bool=newsBiz.deleteNews(id);
if(bool){
count++;
}
}
}
if(count>0){
out.print("<script type='text/javascript'>alert('删除成功!');location.replace('manageNews.jsp')</script>");
}else{
out.print("<script type='text/javascript'>alert('删除失败!');location.replace('manageNews.jsp')</script>");
}