初学，搞不懂哪里出了问题

import openpyxl
from openpyxl import Workbook, load_workbook
wb = load_workbook('mmm.xlsx')
ws = wb.active
for j in range(2, 7):
    hSum = 0
  for i in range(2, 11):
        if ws.cell(i, 2).value == '小学':
            hSum=hSum+ws.cell(i, j + 1).value
#这句总是错误提示。unsupported operand type(s) for +: 'int' and 'NoneType'
        else:
            hSum = 0
  ws.cell(i, j + 1).value = hSum