If I have a fixture with a yield statement within an else clause and the else clause is False, pytest will crash

Here is a simple example (note one of the parametrize lines is purposely commented out as you can use it to toggle back and forth. Only when "filename" is None does it crash. Otherwise it runs fine. Am I missing something? Is there a different way that I'm suppose to do this? My objective is to only have the fixture do something if the filename passed to the fixture is not None. If there is a valid filename, then I want the fixture to do pre-setup work and then yield and then post test cleanup....

import pytest

#################################################
# Simple test
#################################################
#@pytest.mark.parametrize("dut, filename", [("dev1", "myfile.yml"),])
@pytest.mark.parametrize("dut, filename", [("dev1", None),])
def test_1(dut, filename, preconfig_fix2):
    print("Running test function")

@pytest.fixture(scope="function")
def preconfig_fix2(request,):

    filename = request.getfuncargvalue("filename")
    if filename is None:
        print("There is nothing to do here!!")
    else:
        print("There is work to do")
        yield
        print("Finishing up")

python --version
Python 3.7.4

Ubuntu 16.04.6 LTS

pip freeze
alerta==6.5.0
allure-pytest==2.6.5
allure-python-commons==2.6.5
ansi2html==1.5.2
asn1crypto==0.24.0
astroid==2.2.5
atomicwrites==1.3.0
attrs==19.1.0
auto-reporting-system==1.3.3
auto-ssh-client==1.0
auto-tests-fw==1.0
auto-utils==1.0
backcall==0.1.0
backports.ssl==0.0.9
bcrypt==3.1.7
bitarray==1.0.1
blackswan==1.0.75
bleach==3.1.0
certifi==2019.9.11
cffi==1.12.3
chardet==3.0.4
Click==7.0
colorlog==4.0.2
cryptography==2.7
dateparser==0.7.2
datetime-extractor==0.5
decorator==4.1.2
defusedxml==0.6.0
dill==0.3.0
dn-grpc-client==1.0.0
dn-ncclient==1.0.0
docker==3.7.0
docker-pycreds==0.4.0
docutils==0.15.2
dominate==2.3.5
easysnmp==0.2.5
elasticsearch==6.1.1
elasticsearch-dsl==6.1.0
entrypoints==0.3
enum34==1.1.6
Flask==1.0.2
Flask-Bootstrap==3.3.7.1
Flask-Login==0.4.1
flask-nav==0.6
Flask-WTF==0.14.2
future==0.17.1
gevent==1.4.0
GitHub-Flask==3.2.0
googleapis-common-protos==1.5.8
greenlet==0.4.15
grpcio==1.19.0
idna==2.6
importlib-metadata==0.19
ipaddress==1.0.22
ipykernel==5.1.2
ipython==7.8.0
ipython-genutils==0.2.0
ipywidgets==7.5.1
isort==4.3.4
itsdangerous==1.1.0
ixia==1.0.0
IxNetwork==8.50.1501.10
ixnetwork-restpy==1.0.51
jedi==0.15.1
Jinja2==2.10
jira==2.0.0
jsonschema==3.0.2
jupyter==1.0.0
jupyter-client==5.3.3
jupyter-console==6.0.0
jupyter-core==4.5.0
lazy-object-proxy==1.4.2
libraries==1.0
Logbook==1.5.2
lxml==4.1.1
MarkupSafe==1.1.1
mccabe==0.6.1
mistune==0.8.4
more-itertools==7.2.0
natsort==6.0.0
nbconvert==5.6.0
nbformat==4.4.0
ncclient==0.6.4
netaddr==0.7.19
netmiko==2.4.1
notebook==6.0.1
numpy==1.17.2
oauthlib==3.1.0
packaging==19.1
pandas==0.25.1
pandocfilters==1.4.2
paramiko==2.5.0
parso==0.5.1
pbr==5.4.3
pexpect==4.7.0
pickleshare==0.7.5
pluggy==0.12.0
ply==3.11
prometheus-client==0.7.1
prompt-toolkit==2.0.9
protobuf==3.9.1
psycopg2-binary==2.8
ptyprocess==0.6.0
py==1.8.0
pyaml==19.4.1
pyang==2.0.1
pyangbind==0.8.1
pyasn1==0.4.7
pycodestyle==2.5.0
pycparser==2.19
pycryptodomex==3.9.0
pydocstyle==4.0.1
pyflakes==2.1.1
Pygments==2.4.2
pygrok==1.0.0
PyJWT==1.7.1
pylama==7.6.6
pylint==2.3.1
PyNaCl==1.3.0
pyOpenSSL==19.0.0
pyparsing==2.4.2
pyrsistent==0.15.4
pyserial==3.4
pyshark==0.4.2.5
pysmi==0.3.4
pysnmp==4.4.6
pytest==3.7.0
pytest-check==0.3.5
pytest-csv==2.0.1
pytest-dependency==0.4.0
pytest-instafail==0.4.0
pytest-ordering==0.6
pytest-repeat==0.7.0
pytest-reportportal==1.0.5
python-consul==1.1.0
python-dateutil==2.6.1
pytz==2017.3
PyXB==1.2.6
PyYAML==5.1.1
pyzmq==18.1.0
qtconsole==4.5.5
redis==2.10.6
regex==2019.6.8
rejson==0.2.1
reportportal-client==3.2.0
requests==2.18.4
requests-oauthlib==1.2.0
requests-toolbelt==0.9.1
scp==0.13.2
selenium==3.141.0
semantic-version==2.6.0
Send2Trash==1.5.0
six==1.12.0
snowballstemmer==1.9.1
SQLAlchemy==1.3.2
tabulate==0.8.2
terminado==0.8.2
testpath==0.4.2
textfsm==1.1.0
timeout-decorator==0.4.1
tornado==6.0.3
traitlets==4.3.2
typed-ast==1.4.0
tzlocal==2.0.0
urllib3==1.22
visitor==0.1.3
waiting==1.4.1
wcwidth==0.1.7
webencodings==0.5.1
websocket-client==0.56.0
Werkzeug==0.15.6
widgetsnbextension==3.5.1
wrapt==1.11.2
WTForms==2.2.1
WTForms-JSON==0.3.3
XlsxWriter==1.0.2
xmltodict==0.11.0
yangson==1.3.20
zipp==0.5.2