python3中json.dump()和json.load()引发的报错

Question

1.将json.dump() 和json.load()结合使用就会产生如下报错：

import json
# 如果以前存储了用户名，就加载它
#  否则，就提示用户输入用户名并存储它
filename = 'username.json'
try:
    with open(filename) as f_obj:
        username = json.load(f_obj)
except FileNotFoundError:
    username = input("What is your name?")
    filename = 'username.json'
    with open(filename,'w') as f_obj:
        json.dump(username,f_obj)
        print("We'll remember you when you come back, " + username + "!") 
else:
    print("Welcome back, " + username + "!")

报错如下：

百度搜了一下 看不太明白是啥意思 我是初学python照着书上敲的代码 有没有大佬解释下