Can you provide a reproducer? I'm having difficulty reproducing with this script:

import argparse

p = argparse.ArgumentParser()
p.add_argument('--foo', nargs=None)
args = p.parse_args()


$ python3.7 --version
Python 3.7.2
$ python3.7 t.py --foo
usage: t.py [-h] [--foo FOO]
t.py: error: argument --foo: expected one argument