python ntlk donwload给解析器eror

import nltk
nltk.download('all')

但是我收到了这个错误

Traceback (most recent call last):
  File "./update.py",line 3,in <module>
    nltk.download('all')
  File "/usr/lib/python3.6/site-packages/nltk/downloader.py",line 664,in download
    for msg in self.incr_download(info_or_id,download_dir,force):
  File "/usr/lib/python3.6/site-packages/nltk/downloader.py",line 534,in incr_download
    try: info = self._info_or_id(info_or_id)
  File "/usr/lib/python3.6/site-packages/nltk/downloader.py",line 508,in _info_or_id
    return self.info(info_or_id)
  File "/usr/lib/python3.6/site-packages/nltk/downloader.py",line 875,in info
    self._update_index()
  File "/usr/lib/python3.6/site-packages/nltk/downloader.py",line 825,in _update_index
    ElementTree.parse(compat.urlopen(self._url)).getroot())
  File "/usr/lib/python3.6/xml/etree/ElementTree.py",line 1196,in parse
    tree.parse(source,parser)
  File "/usr/lib/python3.6/xml/etree/ElementTree.py",line 597,in parse
    self._root = parser._parse_whole(source)
xml.etree.ElementTree.ParseError: not well-formed (invalid token): line 23,column 143

我是python的新手,所以我不确定我该怎么办.
我查看了上面报告的源模块,发现它正在尝试下载xml文件.所以我运行下面的命令,并没有给我任何错误.

compat.urlopen('https://raw.githubusercontent.com/nltk/nltk_data/gh-pages/index.xml')

所以我认为下载中没有问题,但在解析器中.有人可以建议我如何从这里开始？