brew cask のインストールでエラー

$ brew cask install firefox-ja
==> Downloading https://download.mozilla.org/?product=firefox-latest&os=osx&lang
Already downloaded: /Library/Caches/Homebrew/firefox-ja-latest
Error: undefined method `extname' for nil:NilClass
Please report this bug:
    https://github.com/caskroom/homebrew-cask/issues
/usr/local/Cellar/brew-cask/0.48.0/rubylib/cask/container/criteria.rb:43:in `extension'
/usr/local/Cellar/brew-cask/0.48.0/rubylib/cask/container/pkg.rb:3:in `me?'
/usr/local/Cellar/brew-cask/0.48.0/rubylib/cask/container.rb:50:in `block in for_path'
/usr/local/Cellar/brew-cask/0.48.0/rubylib/cask/container.rb:48:in `each'
/usr/local/Cellar/brew-cask/0.48.0/rubylib/cask/container.rb:48:in `find'
/usr/local/Cellar/brew-cask/0.48.0/rubylib/cask/container.rb:48:in `for_path'
/usr/local/Cellar/brew-cask/0.48.0/rubylib/cask/installer.rb:83:in `extract_primary_container'
/usr/local/Cellar/brew-cask/0.48.0/rubylib/cask/installer.rb:50:in `install'
/usr/local/Cellar/brew-cask/0.48.0/rubylib/cask/cli/install.rb:20:in `block in install_casks'
/usr/local/Cellar/brew-cask/0.48.0/rubylib/cask/cli/install.rb:17:in `each'
/usr/local/Cellar/brew-cask/0.48.0/rubylib/cask/cli/install.rb:17:in `install_casks'
/usr/local/Cellar/brew-cask/0.48.0/rubylib/cask/cli/install.rb:6:in `run'
/usr/local/Cellar/brew-cask/0.48.0/rubylib/cask/cli.rb:81:in `run_command'
/usr/local/Cellar/brew-cask/0.48.0/rubylib/cask/cli.rb:121:in `process'
/usr/local/bin/brew-cask.rb:42:in `<top (required)>'
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
/usr/local/Library/brew.rb:59:in `require?'
/usr/local/Library/brew.rb:142:in `<main>'

以下を実行したらインストール出来た。

$ brew update && brew upgrade brew-cask && brew cleanup && brew cask cleanup

実際は、brew updateだけはしてあったのだが、brew upgrade brew-caskが重要なのかしら？

参考