190 Query String Parameters

Hi. Can you help me? I deployed app for this task on Heroku.
It uses automatic HTTPS redirect unless it's manually specified.
I use Flask for my app and manually specify HTTP in scheme param for the POST request from the form.
But for some reason I keep getting this error despite the fact that my application is serving the page with HTTP:

Second url should start with http://

There is my app: http://volumeletters.herokuapp.com/querystring/