git push heroku masterができない。LoadError: Could not load the 'mysql' Active Record adapter.

前提・実現したいこと

自作したRailsアプリケーションをHerokuにデプロイしたいのですが、エラーに対応できず困っています。
git push heroku masterをすると
LoadError: Could not load the 'mysql' Active Record adapter.
が出るのですが、gemfileに'mysql2'はインストールされていますし、database.ymlに記述したadapter: mysql2も間違っていないと思い、他にどのように対応するべきかご教授いただけると幸いです。

発生している問題・エラーメッセージ

$ git push heroku master
Enumerating objects: 642, done.
Counting objects: 100% (642/642), done.
Delta compression using up to 4 threads
Compressing objects: 100% (549/549), done.
Writing objects: 100% (590/590), 3.91 MiB | 302.00 KiB/s, done.
Total 590 (delta 356), reused 5 (delta 0)
remote: Compressing source files... done.
remote: Building source:
remote:
remote: -----> Ruby app detected

(中略)

remote:        Cleaning up the bundler cache.
remote: -----> Installing node-v10.14.1-linux-x64
remote: -----> Detecting rake tasks
remote: -----> Preparing app for Rails asset pipeline
remote:        Running: rake assets:precompile
remote:        rake aborted!
remote:        LoadError: Could not load the 'mysql' Active Record adapter. Ensure that the adapter is spelled correctly in config/database.yml and that you've added the necessary adapter gem to your Gemfile.
remote:        /tmp/build_fffb337257033326389fb1b6bf80cc2b/vendor/bundle/ruby/2.6.0/gems/bootsnap-1.4.4/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:33:in `require'
remote:        /tmp/build_fffb337257033326389fb1b6bf80cc2b/vendor/bundle/ruby/2.6.0/gems/activesupport-5.2.3/lib/active_support/dependencies.rb:291:in `block in require'
remote:        /tmp/build_fffb337257033326389fb1b6bf80cc2b/vendor/bundle/ruby/2.6.0/gems/activesupport-5.2.3/lib/active_support/dependencies.rb:257:in `load_dependency'
remote:        /tmp/build_fffb337257033326389fb1b6bf80cc2b/vendor/bundle/ruby/2.6.0/gems/activesupport-5.2.3/lib/active_support/dependencies.rb:291:in `require'
remote:        /tmp/build_fffb337257033326389fb1b6bf80cc2b/vendor/bundle/ruby/2.6.0/gems/activerecord-5.2.3/lib/active_record/connection_adapters/connection_specification.rb:19

該当のソースコード

config/database.yml
default: &default
  adapter: mysql2
  encoding: utf8
  reconnect: false
  pool: 5

development:
  <<: *default
  database: データベース名
  username: root
  password:
  host: localhost

# Warning: The database defined as "test" will be erased and
# re-generated from your development database when you run "rake".
# Do not set this db to the same as development or production.

test:
  <<: *default
  database: データベース名
  username: root
  password:
  host: localhost

production:
  <<: *default
  database: heroku_で始まるデータベース名
  username: ユーザ名
  password: <%= ENV['DATABASE_PASSWORD'] %>

Gemfile

source 'https://rubygems.org'
git_source(:github) { |repo| "https://github.com/#{repo}.git" }

ruby '2.6.2'

# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '~> 5.2.3'
# Use mysql2 as the database for Active Record
gem 'mysql2'
# Use Puma as the app server
gem 'puma', '~> 3.11'
# Use SCSS for stylesheets

（後略）

Gemfile.lock

    activerecord-mysql2-adapter (0.0.3)
      mysql2

    mysql2 (0.5.2)

（抜粋）