I accidentally stumbled upon this bug when I used -m instead of --pyargs in one of my projects:
https://gist.github.com/boustrophedon/338a6572e52d6d44c9175783e9657209

Environment: Linux, Python 3.7 and Pytest 3.8.1 both via pipenv

To reproduce, create a directory structure that looks like:

repro_case/
repro_case/a/
repro_case/a/test_a.py

The contents of "a/test_a.py" are

def test_a():
  pass

In the repro_case directory, running pytest by itself passes the trivial test, but running pytest -m a/ crashes and produces the linked error.

The crash does not occur if there is no test_a.py file, or if the test_a.py file is empty.

pip list inside the virtualenv produces:
Package Version

atomicwrites 1.2.1
attrs 18.2.0
more-itertools 4.3.0
pip 18.0
pluggy 0.7.1
py 1.6.0
pytest 3.8.1
setuptools 40.4.3
six 1.11.0
wheel 0.31.1