diff --git a/Library/Homebrew/cask/installer.rb b/Library/Homebrew/cask/installer.rb
index 5ad75d01ce..e28e4fa093 100644
--- a/Library/Homebrew/cask/installer.rb
+++ b/Library/Homebrew/cask/installer.rb
@@ -13,6 +13,9 @@ require "cask/quarantine"
 require "cgi"
 
 module Cask
+  # Installer for a {Cask}.
+  #
+  # @api private
   class Installer
     extend Predicable
     # TODO: it is unwise for Cask::Staged to be a module, when we are