Ошибка TypeError: cannot serialize (type builtin_function_or_method)

У меня есть такой код

from bs4 import BeautifulSoup
import xml.etree.ElementTree as ET
import requests


def htmlparser(input_file_path, output_file_path):
    with open(input_file_path, 'r', encoding='utf-8') as f1:
        with open(output_file_path, 'w', encoding='utf-8') as f2:
            f2.write('<?xml version="1.0" encoding="UTF-8"?>')
            documents = ET.Element('documents')
            for url in f1:
                document = ET.SubElement(documents, 'document')
                document.set('id ', url.index)
                url_ = ET.SubElement(document, 'url')
                url_.text = url

                html = requests.get(url).text

                soup = BeautifulSoup(html, 'html.parser')
                soup_keywords = BeautifulSoup(html, 'html5lib')

                title = soup.find(class_="article__title")
                title_ = ET.SubElement(document, 'title')
                title_.text = title.string

                content = soup.findAll(class_="article__text")

                for item in content:
                    if item.string is not None:
                        full_text = item.string + ' '

                text_ = ET.SubElement(document, 'text')
                text_.text = full_text

                keywords = soup_keywords.find(attrs={'name': 'keywords'})["content"]

                keywords_ = ET.SubElement(document, 'keywords')
                keywords_.text = keywords

            data = ET.tostring(documents, encoding='utf8')
            f2.write(data)


htmlparser('input.csv', 'output.xml')

Каждый раз Python3 выдает ошибку

Traceback (most recent call last):
  File "C:\Users\user\AppData\Local\Programs\Python\Python36-32\lib\xml\etree\ElementTree.py", line 1079, in _escape_attrib
    if "&" in text:
TypeError: argument of type 'builtin_function_or_method' is not iterable

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:/**/HTMLparser.py", line 45, in <module>
    htmlparser('input.csv', 'output.xml')
  File "C:/**/HTMLparser.py", line 41, in htmlparser
    data = ET.tostring(documents, encoding='utf8')
  File "C:\Users\user\AppData\Local\Programs\Python\Python36-32\lib\xml\etree\ElementTree.py", line 1135, in tostring
    short_empty_elements=short_empty_elements)
  File "C:\Users\user\AppData\Local\Programs\Python\Python36-32\lib\xml\etree\ElementTree.py", line 776, in write
    short_empty_elements=short_empty_elements)
  File "C:\Users\user\AppData\Local\Programs\Python\Python36-32\lib\xml\etree\ElementTree.py", line 941, in _serialize_xml
    short_empty_elements=short_empty_elements)
  File "C:\Users\user\AppData\Local\Programs\Python\Python36-32\lib\xml\etree\ElementTree.py", line 933, in _serialize_xml
    v = _escape_attrib(v)
  File "C:\Users\user\AppData\Local\Programs\Python\Python36-32\lib\xml\etree\ElementTree.py", line 1102, in _escape_attrib
    _raise_serialization_error(text)
  File "C:\Users\user\AppData\Local\Programs\Python\Python36-32\lib\xml\etree\ElementTree.py", line 1057, in _raise_serialization_error
    "cannot serialize %r (type %s)" % (text, type(text).__name__)
TypeError: cannot serialize <built-in method index of str object at 0x0908FA20> (type builtin_function_or_method)

Подскажите, пожалуйста, в чем проблема?