diff --git a/Library/Homebrew/formula_installer.rb b/Library/Homebrew/formula_installer.rb
index 1e07bdbaa0d3ca7b016818c8835603310f26af29..2cc00eaff097e65d2d673aea135177410fff6622 100644
--- a/Library/Homebrew/formula_installer.rb
+++ b/Library/Homebrew/formula_installer.rb
@@ -19,6 +19,7 @@ class FormulaInstaller
 
   def self.mode_attr_accessor(*names)
     attr_accessor(*names)
+    private(*names)
     names.each { |name| define_method("#{name}?") { !!send(name) }}
   end