c#中遇到NullReferenceException异常，求大神

                student s;
                string x = "11111111";
                    s._number = x;
                s._name = "曾艳";
                s._money = 200;
                s._category = 1;
                users.Add(s._number, s);
提示NullReferenceException异常，
  private Dictionary<string,student> users;     
 private struct student{
            public int _category;
            public string _number;
            public string _name;
            public int _money;
        }