SqlDataReader reader = cmd.ExecuteReader();一直报错这个哪里出问题了啊？

SqlDataReader reader = cmd.ExecuteReader();一直报错这个哪里出问题了啊求教。

private void button1_Click(object sender, EventArgs e)
        {
            if (student.Checked)
            {
                using (SqlConnection conn = new SqlConnection("Data Source=ICARUS;Integrated Security=SSPI;Initial Catalog=STUDENT"))
                {
                    conn.Open();
                    String sql = "select * from dbo.Student where StuLogic=@un and StuPW=@pw";
                    SqlCommand cmd = new SqlCommand(sql, conn);
                    cmd.Parameters.Add(new SqlParameter("@un", username));
                    cmd.Parameters.Add(new SqlParameter("@pw", password));
                    SqlDataReader reader = cmd.ExecuteReader();
                    if (reader.Read())
                    {
                        Student stu = new Student();
                        stu.Show();
                        this.Hide();
                    }
                    else {
                        MessageBox.Show("请重新登录！");
                    }
                }
            }
        }

cmd.Parameters.Add(new SqlParameter("@un", SqlDbType.NVarChar, xxx));
cmd.Parameters.Add(new SqlParameter("@pw", SqlDbType.NVarChar, xxx));
cmd.Parameters["@un"].Value = username;
cmd.Parameters["@pw"].Value = password;
其中xxx是你的数据库里字段长度