Following the Flask 0.11 documentation, I cloned the Flask repo, created a virtualenv, and installed Flask via pip install flask. I went into the example
pip install flask
example
I was setting FLASK_APP=flaskr. Since I hadn't installed my app as a package in my virtualenv, I needed to add the .py extension.
FLASK_APP=flaskr
.py
export FLASK_APP=flaskr.py