位置情報に PostGIS を使用する Rails アプリケーションを Heroku にデプロイしようとしています。データベースへの PostGIS ( https://devcenter.heroku.com/articles/postgis )のインストールに関する Heroku のドキュメントに従いましたが、spatial_adapter gem から次のエラーが発生しました。
/app/vendor/bundle/ruby/1.9.1/gems/spatial_adapter-1.2.0/lib/spatial_adapter/railtie.rb:4:in `block in <class:Railtie>': undefined method `[]' for nil:NilClass (NoMethodError)
from /app/vendor/bundle/ruby/1.9.1/gems/railties-3.2.13/lib/rails/initializable.rb:30:in `instance_exec'
from /app/vendor/bundle/ruby/1.9.1/gems/railties-3.2.13/lib/rails/initializable.rb:30:in `run'
from /app/vendor/bundle/ruby/1.9.1/gems/railties-3.2.13/lib/rails/initializable.rb:55:in `block in run_initializers'
from /app/vendor/bundle/ruby/1.9.1/gems/railties-3.2.13/lib/rails/initializable.rb:54:in `each'
from /app/vendor/bundle/ruby/1.9.1/gems/railties-3.2.13/lib/rails/initializable.rb:54:in `run_initializers'
from /app/vendor/bundle/ruby/1.9.1/gems/railties-3.2.13/lib/rails/application.rb:136:in `initialize!'
from /app/vendor/bundle/ruby/1.9.1/gems/railties-3.2.13/lib/rails/railtie/configurable.rb:30:in `method_missing'
from /app/config/environment.rb:5:in `<top (required)>'
from /app/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.13/lib/active_support/dependencies.rb:251:in `require'
from /app/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.13/lib/active_support/dependencies.rb:251:in `block in require'
from /app/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.13/lib/active_support/dependencies.rb:236:in `load_dependency'
from /app/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.13/lib/active_support/dependencies.rb:251:in `require'
from /app/vendor/bundle/ruby/1.9.1/gems/railties-3.2.13/lib/rails/application.rb:103:in `require_environment!'
from /app/vendor/bundle/ruby/1.9.1/gems/railties-3.2.13/lib/rails/commands.rb:40:in `<top (required)>'
from script/rails:6:in `require'
from script/rails:6:in `<main>'
このエラーの原因について何か考えはありますか? 前もって感謝します！