Still having problems with "nill error" after running suggested command

[mAAdhaTTah]

I'm still having the #7946 problem after running the suggested command:

○ → brew update && brew upgrade brew-cask && brew cleanup && brew cask cleanup
Already up-to-date.
Error: brew-cask-HEAD already installed

○ → brew cask install virtualbox --force

==> Downloading http://download.virtualbox.org/virtualbox/4.3.20/VirtualBox-4.3.20-96996-OSX.dmg
Already downloaded: /Library/Caches/Homebrew/virtualbox-4.3.20-96996.dmg
Error: no implicit conversion of nil into String
Please report this bug:
    https://github.com/caskroom/homebrew-cask/issues
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/digest.rb:44:in `initialize'
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/digest.rb:44:in `open'
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/digest.rb:44:in `file'
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/digest.rb:29:in `file'
/usr/local/Cellar/brew-cask/HEAD/rubylib/cask/download.rb:41:in `block in _check_sums'
/usr/local/Cellar/brew-cask/HEAD/rubylib/cask/download.rb:39:in `each'
/usr/local/Cellar/brew-cask/HEAD/rubylib/cask/download.rb:39:in `_check_sums'
/usr/local/Cellar/brew-cask/HEAD/rubylib/cask/download.rb:32:in `perform'
/usr/local/Cellar/brew-cask/HEAD/rubylib/cask/installer.rb:54:in `download'
/usr/local/Cellar/brew-cask/HEAD/rubylib/cask/installer.rb:31:in `install'
/usr/local/Cellar/brew-cask/HEAD/rubylib/cask/cli/install.rb:20:in `block in install_casks'
/usr/local/Cellar/brew-cask/HEAD/rubylib/cask/cli/install.rb:17:in `each'
/usr/local/Cellar/brew-cask/HEAD/rubylib/cask/cli/install.rb:17:in `install_casks'
/usr/local/Cellar/brew-cask/HEAD/rubylib/cask/cli/install.rb:6:in `run'
/usr/local/Cellar/brew-cask/HEAD/rubylib/cask/cli.rb:80:in `run_command'
/usr/local/Cellar/brew-cask/HEAD/rubylib/cask/cli.rb:120: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>'

[vitorgalvao]

Explanation. Possible resolution.

[mAAdhaTTah]

That did it. Thanks!