Railsでwebpackerを入れるとherokuへのデプロイでこける

環境

Rails 5.1.5
webpack 3.11.0

エラーメッセージ

remote:        Webpacker is installed  
remote:        Using /tmp/build_a60546bb726d2fa7f4b8b4ac052f461e/config/webpacker.yml file for setting up webpack paths
remote:        Compiling…
remote:        Compilation failed:
remote:        bundler: failed to load command: webpack (/tmp/build_a60546bb726d2fa7f4b8b4ac052f461e/vendor/bundle/ruby/2.3.0/bin/webpack)
remote:        Errno::ENOENT: No such file or directory - /tmp/build_a60546bb726d2fa7f4b8b4ac052f461e/node_modules/.bin/webpack