assign None to Series got ValueError

[hack4code]

import pandas as pd

s = pd.Series({'aa': 1})
s['aa'] = None

Problem description

got exception:
ValueError: ['a' 'a'] not contained in the index

Expected Output

Output of pd.show_versions()

INSTALLED VERSIONS

commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Darwin
OS-release: 17.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.21.0
pytest: None
pip: 9.0.1
setuptools: 28.8.0
Cython: None
numpy: 1.13.1
scipy: 0.19.1
pyarrow: None
xarray: None
IPython: 6.0.0
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.999999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

[chris-b1]

Strange, somewhere in the indexing code 'aa' gets treated as a iterable. PR would be welcome. Note that it would be more typical to use np.nan as a missing value, which is handled correctly (s['aa'] = np.nan)

[fredrik-1]

The problem is that _set_with_engine throws a typeError, _set_with is used instead which for some reason change the key to a list with key = list(key) and list('aa')=['a','a']

So one solution might be to change key=list(key) with

if isinstance(key, str):
     key=[key]
else:
     key = list(key)

I don't know if this works in both python 2 and 3

[JustinZhengBC]

Solved in #23495