mailbox.MH crashes on certain Claws Mail .mh_sequences files

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-10-18.22:39:26.549>
labels = ['3.7', 'type-feature', 'library']
title = 'mailbox.MH crashes on certain Claws Mail .mh_sequences files'
updated_at = <Date 2016-10-29.18:47:04.178>
user = 'https://bugs.python.org/garrettzilla'

bugs.python.org fields:

activity = <Date 2016-10-29.18:47:04.178>
actor = 'r.david.murray'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2016-10-18.22:39:26.549>
creator = 'garrettzilla'
dependencies = []
files = []
hgrepos = []
issue_num = 28473
keywords = []
message_count = 7.0
messages = ['278945', '278954', '278955', '278972', '279683', '279684', '279693']
nosy_count = 2.0
nosy_names = ['r.david.murray', 'garrettzilla']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue28473'
versions = ['Python 3.6', 'Python 3.7']