Unstable result of unsubscriptable-object error within if/elif/else block

[eight04]

Steps to reproduce

Create following files (or unzip sample.zip): test.py

a = None
for i in range(1, 5):
    if i % 2:
        a = "foo"
    else:
        a = a[1:]

test2.py

a = None
for i in range(1, 5):
    if i % 2:
        a = "foo"
    if not i % 2:
        a = a[1:]

test3.py

a = None
for i in range(1, 5):
    if i % 2:
        a = "foo"
    elif not i % 2:
        a = a[1:]

Then run:

pylint --disable=all --enable=typecheck test.py test2.py test3.py

Current behavior

No config file found, using default configuration
************* Module test3
E:  6, 6: Value 'a' is unsubscriptable (unsubscriptable-object)

Expected behavior

I think all of them should report the error, otherwise all of them should not. I'm not sure what is the difference between if/elif/else in pylint. Should I rewrite my code to avoid elif in this case?

A live example: https://github.com/eight04/pyAPNG/blob/bc5b9d9da666bbcdfa65ed3512134a9d143cb650/apng/init.py#L376

pylint --version output

D:\Dev\pyAPNG>pylint --version
Using config file D:\Dev\pyAPNG\.pylintrc
pylint 1.8.4,
astroid 1.6.1
Python 3.6.0 (v3.6.0:41df79263a11, Dec 23 2016, 07:18:10) [MSC v.1900 32 bit (In
tel)]