[C#] Code Error ตรง com.ExecuteNonQuery(); ครับ ช่วยทีครับ ขอบคุณครับ

 private void ManageStoge()
        {
             conn = new SqlConnection(Program.strConn);
            conn.Open();
            string sql2 = "SELECT PID,Quantity FROM Sale WHERE PID =  @PID";
            for (int i = 0; i <= dataGridView2.Rows.Count - 2; i++)
            {
                com = new SqlCommand(sql2, conn);
                com.Parameters.Clear();
                com.Parameters.AddWithValue("@PID", dataGridView2.Rows[i].Cells[0].Value);
                dr = com.ExecuteReader();

                while (dr.Read())
                {
                    string sql = "Update Product set UnitInStock = UnitInStock - '" + dr["Quantity"].ToString() + "' WHERE PID = '" + dr["PID"].ToString() + "'";
                    //string strCom = "UPDATE product SET STOCK = STOCK - '"+reader["Num"].ToString()+"' WHERE PRODUCT_ID = '"+reader["PRODUCT_ID"].ToString()+"'";
                    com = new SqlCommand(sql, conn);
                                      com.ExecuteNonQuery(); //  <<<<<<<<< Error ตรงนี้ครับ
                }
            }
             
            dr.Close();
            conn.Close();
        }