operator.concat/iconcat could only work if left operand is a sequence

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-01-03.09:58:30.441>
labels = ['3.7', 'type-bug', 'library']
title = 'operator.concat/iconcat could only work if left operand is a sequence'
updated_at = <Date 2017-03-18.06:26:19.039>
user = 'https://github.com/zhangyangyu'

bugs.python.org fields:

activity = <Date 2017-03-18.06:26:19.039>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-01-03.09:58:30.441>
creator = 'xiang.zhang'
dependencies = []
files = []
hgrepos = []
issue_num = 29139
keywords = []
message_count = 6.0
messages = ['284546', '284547', '284549', '284577', '284581', '289792']
nosy_count = 5.0
nosy_names = ['rhettinger', 'vstinner', 'zach.ware', 'serhiy.storchaka', 'xiang.zhang']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue29139'
versions = ['Python 3.7']