运行的时候一直提示是 not contain a loop with signa?400报错

请教各位大侠，运行程序的时候，一直提示报错:

Traceback (most recent call last):
  File "C:/Python_3.6/Python_File_Py/EX_001/003.py", line 14, in <module>
    address = house_num + ' ' + street + "," + city
TypeError: ufunc 'add' did not contain a loop with signature matching types dtype('<U21') dtype('<U21') dtype('<U21')

代码如下:

import xlrd  # 导入xlrd模块
import requests  # 导入requests模块
from datetime import date, datetime  # 从模块datetime中导入函数date,datetime
import pandas as pd  # 导入pandas大熊猫模块，并将其命名为pd
from math import sin, asin, cos, radians, fabs, sqrt

loc_list = [],
df = pd.read_excel('C:\Python_3.6\Python_File_Py\EX_001\\KS_01.xlsx')
rows = df.iloc[:, 0].size  # 行数 3
for indexs in df.index:
    street = df.loc[indexs].values[0]
    house_num = df.loc[indexs].values[1]
    city = df.loc[indexs].values[2]
    address = house_num + ' ' + street + "," + city

print(address)

请问问题出在哪里啊