Automatic upcast does not occur for custom classes

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-06-17.19:29:40.309>
labels = ['ctypes', 'type-bug']
title = 'Automatic upcast does not occur for custom classes'
updated_at = <Date 2017-06-17.19:39:41.287>
user = 'https://bugs.python.org/rharke'

bugs.python.org fields:

activity = <Date 2017-06-17.19:39:41.287>
actor = 'ned.deily'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['ctypes']
creation = <Date 2017-06-17.19:29:40.309>
creator = 'rharke'
dependencies = []
files = []
hgrepos = []
issue_num = 30692
keywords = []
message_count = 1.0
messages = ['296245']
nosy_count = 4.0
nosy_names = ['amaury.forgeotdarc', 'belopolsky', 'meador.inge', 'rharke']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue30692'
versions = ['Python 2.7', 'Python 3.6']