使用umwrap报错：TypeError: 'NoneType' object is not callable
代码：

encoding: utf-8

2 from bs4 import BeautifulSoup
3 make = 'I linked to example.com'
4 soup = BeautifulSoup(make)
5 a_tag = soup.a
6 a_tag.i.unwrap()
7 print a_tag

i类型是none，没有获取到。