Map遍历增加key报错如何解决

public static void main(String[] args) throws Exception{
      Map<String,Object> aa=new HashMap<String, Object>();
      aa.put("a1","a1");
      aa.put("a2","a2");
      aa.put("a3","a3");
      aa.put("a4","a4");
      Set<String>  ss=aa.keySet();
      Iterator it=ss.iterator();
      while(it.hasNext()){
        Object key=it.next();  
        if("a1".equals(key)){
           aa.put("aa1","aa1");
        }
      }
    System.out.println(aa);
}
报错如下：

Exception in thread "main" java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextNode(HashMap.java:1437)
at java.util.HashMap$KeyIterator.next(HashMap.java:1461)