Weird “is_xhr” error when deploying Flask app to Heroku

The Werkzeug library (dependency from Flask) recently received a major update (0.16.1 –> 1.0.0) and it looks like Flask (<=0.12.4) does not restrict the version of Werkzeug that is fetched.

You have 2 options:

• Stick with your current version of Flask and restrict the Werkzeug version that is fetched explicitly in your application’s setup.py or requirements.txt by specifying werkzeug<1.0 or werkzeug==0.16.1

• Upgrade to a recent version of Flask (>=1.0.0), which is running fine with latest Werkzeug