我正在尝试求出城市表中所有人口的平均值。我正在进行一个项目，下面是说明。我已经完成了第一步：

当我试图从“cities”表中获取“city_population“并将其除以列中的项时，我得到了一个错误。这是我的代码：

import sqlite3
import os 

# Remove Database file if it exists:

os.remove('cities.db')

connection = sqlite3.connect("cities.db")
cursor = connection.cursor()

cursor.execute("create table if not exists cities(city_id INTEGER PRIMARY KEY NOT NULL, cities_name text, city_population integer)")
    
cities_list = [('Minneapolis', '425,336'),
    ('St. Paul', '307,193'),
    ('Dallas', '1,288,000'),
    ('Memphis', '628, 127'),
    ('San Francisco', '815,201'),
    ('Milwaukee', '569,330'),
    ('Denver', '711,463'),
    ('Phoenix', '1,625,000'),
    ('Chicago', '2,697,000'),
    ('New York', '8,468,000')]

cursor.executemany("INSERT OR IGNORE INTO cities VALUES (NULL, ?, ?)", cities_list)

# Print cities in alphabetical order

cursor.execute("select cities_name from cities")
result = sorted(cursor.fetchall())
print(result)

# Print average (NEED HELP HERE!)

cursor.execute("select city_population from cities")
result = cursor.fetchall()

average = sum(result) / len(result)
print(average)

connection.commit()

connection.close()

我希望显示总体的平均值，但我得到了一个错误。我该如何修复它？