我在添加单链表的元素时遇到运行时错误。我在这里输入了错误图片的图像描述。我在这里附上我的代码：

public class Node{
      Node next;
      int a=0;

        public Node (int e, Node n){
          a =e;
          next =n;
      }}

    public class Question6{
      static int sum=0;
      public static void main(String[]args){

        Node head=null;

        Node n5= new Node(5,null);
        Node n4= new Node(4,n5);
        Node n3= new Node(3,n4);
        Node n2= new Node(2,n3);
        Node n1= new Node(1,n2);

        head=n1;
        sum =add(head);   
      }
      public static int add(Node head){
        if(head==null){
          return sum;
        }
        else{
          sum=sum + head.a;
          head=head.next;
          add(head);
        }
        return sum;
      }
    }