No output, even with `py.test -s`

I would like to use py.test combined with hunter:

PYTHONHUNTER="module_startswith='foo'" py.test -s -k test_bar

Unfortunately the output (trace) of hunter is not visible.

Version:

foo_cok_d@aptguettler:~$ py.test --version

This is pytest version 3.4.2, imported from /home/foo_cok_d/local/lib/python2.7/site-packages/pytest.pyc
setuptools registered plugins:
  pytest-xdist-1.22.2 at /home/foo_cok_d/local/lib/python2.7/site-packages/xdist/looponfail.py
  pytest-xdist-1.22.2 at /home/foo_cok_d/local/lib/python2.7/site-packages/xdist/plugin.py
  pytest-forked-0.2 at /home/foo_cok_d/local/lib/python2.7/site-packages/pytest_forked/__init__.pyc

In a simpler (smaller) virtualenv it works (same pytest version, but no plugins).

What could be the reason?

How to debug this?

from No output, even with `py.test -s`