I can't migrate the database of my Rails app on Windows 10

I'm following this tutorial on creating Ruby on Rail apps, and I've hit a roadblock. I am trying to migrate its database to Heroku, only to be told this:

c:\Sites\railsbridge\test_app>heroku run rails db:migrate
Running rails db:migrate on floating-ocean-40882... up, run.5709 (Free)
rails aborted!
Gem::LoadError: Specified 'sqlite3' for database adapter, but the gem is not 
loaded. Add `gem 'sqlite3'` to your Gemfile (and ensure its version is at the 
minimum required by ActiveRecord).

I already have sqlite3 listed in my Gemfile, what am I doing wrong?