爬取地理网站的图片报错raise MissingSchema(error) requests.exceptions.MissingSchema: Invalid URL '': No schema supplied. Perhaps you meant http://?

[whiteminta3]

from bs4 import BeautifulSoup
import requests
import os
os.makedirs('./img/', exist_ok=True)

URL = "http://www.nationalgeographic.com.cn/animals/"

html = requests.get(URL).text
soup = BeautifulSoup(html, 'lxml')
img_li = soup.find_all('li', {"class": "mod_w"})

for li in img_li:
imgs = li.find_all('img')
for img in imgs:
url = img['src']
r = requests.get(url, stream=True)
image_name = url.split('/')[-1]
with open('./img/%s' % image_name, 'wb') as f:
for chunk in r.iter_content(chunk_size=128):
f.write(chunk)
print('Saved %s' % image_name)

[whiteminta3]

这是运行后报错的代码
Traceback (most recent call last):
File "3-3-practice-download-images.py", line 16, in
r = requests.get(url, stream=True)
File "C:\PYTHON\lib\site-packages\requests\api.py", line 76, in get
return request('get', url, params=params, **kwargs)
File "C:\PYTHON\lib\site-packages\requests\api.py", line 61, in request
return session.request(method=method, url=url, **kwargs)
File "C:\PYTHON\lib\site-packages\requests\sessions.py", line 516, in request
prep = self.prepare_request(req)
File "C:\PYTHON\lib\site-packages\requests\sessions.py", line 449, in prepare_request
p.prepare(
File "C:\PYTHON\lib\site-packages\requests\models.py", line 314, in prepare
self.prepare_url(url, params)
File "C:\PYTHON\lib\site-packages\requests\models.py", line 388, in prepare_url
raise MissingSchema(error)
requests.exceptions.MissingSchema: Invalid URL '': No schema supplied. Perhaps you meant http://?