email.contentmanager raises error when policy.max_line_length==None or 0

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-09-25.15:36:08.767>
labels = ['3.8', 'type-bug', '3.7', 'expert-email']
title = 'email.contentmanager raises error when policy.max_line_length==None or 0'
updated_at = <Date 2018-09-25.20:24:05.900>
user = 'https://github.com/silane'

bugs.python.org fields:

activity = <Date 2018-09-25.20:24:05.900>
actor = 'silane'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['email']
creation = <Date 2018-09-25.15:36:08.767>
creator = 'silane'
dependencies = []
files = []
hgrepos = []
issue_num = 34800
keywords = ['patch']
message_count = 4.0
messages = ['326364', '326384', '326399', '326403']
nosy_count = 4.0
nosy_names = ['barry', 'r.david.murray', 'xtreak', 'silane']
pr_nums = ['9578']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue34800'
versions = ['Python 3.7', 'Python 3.8']