Python的json.load发生异常: JSONDecodeError Expecting value: line 1 column 1 (char 0)

import json
filename = 'username.json'
try:
    with open(filename) as f:        
        username=json.load(f)        
except FileNotFoundError:
    username = input("qin_shu_ru")
    with open(filename,'w') as f:
        json.dump(username,f)
        print(f"{username}")
else:
    print(f"{username}")

照着书上打的为什么 username=json.load(f)这行会有问题

username.json 里是啥， 可能有KEY 没用引号。