【Python】エラーについて

Pythonのエラーについて
以下のエラーの対処方法が分かりません。
どうか、ご教授ください。

発生している問題・エラーメッセージ

Traceback (most recent call last):
  File "aozora_bunko_xml_exporter.py", line 79, in <module>
    xml_str = create_xml()
  File "aozora_bunko_xml_exporter.py", line 73, in create_xml
    = work.mojidukai_type.name
AttributeError: 'NoneType' object has no attribute 'name'

該当のソースコード

import xml.etree.ElementTree as ElementTree
from xml.dom import  minidom
import logging

from orator import DatabaseManager, Model
from orator.orm import belongs_to, has_many

logger = logging.getLogger('orator.connection.queries')
logger.setLevel(logging.DEBUG)

formatter = logging.Formatter(
	'It took %(elapsed_time)sms to execute the query %(query)s'
)

handler = logging.StreamHandler()
handler.setFormatter(formatter)

logger.addHandler(handler)

config = {
	'mysql': {
		'driver': 'mysql',
		'host': 'localhost',
		'database': 'aozora_bunko',
		'user': 'root',
		'password': '',
		'prefix': '',
		'log_queries': True,
	}
}

db = DatabaseManager(config)
Model.set_connection_resolver(db)

class MojidukaiType(Model):
	
	pass

class Work(Model):

	URL_FORMAT = "http://www.aozora_bunko.gr.jp/cards/{writer_id:06d}/card{id}.html"

	@belongs_to
	def mojidukai_type(self):
		return MojidukaiType

	@belongs_to
	def writer(self):
		return writer

	def build_url(self):
		return self.URL_FORMAT.format(
			writer_id=self.writer_id,
			id=self.id
		)		

class Writer(Model):
	
	@has_many
	def works(self):
		return Work

def create_xml():
	elm_root = ElementTree.Element("catalog")
	writers = Writer.all()
	writers.load('works', 'works.mojidukai_type')
	for writer in writers:
		for work in writer.works:
			elm_work = ElementTree.SubElement(elm_root, "work", id=str(work.id))
			ElementTree.SubElement(elm_work, "writer", id=str(writer.id)).text = writer.name
			ElementTree.SubElement(elm_work, "title").text = work.title
			ElementTree.SubElement(elm_work, "mojidukai_type", id=str(work.mojidukai_type.id)).text \
			         = work.mojidukai_type.name
			ElementTree.SubElement(elm_work, "url").text = work.build_url()
	with minidom.parseString(ElementTree.tostring(elm_root, 'utf-8')) as dom:
		return dom.toprettyxml(indent="  ")

if __name__ == '__main__':
	xml_str = create_xml()
	print(xml_str)