diff --git a/Library/Homebrew/formula_installer.rb b/Library/Homebrew/formula_installer.rb
index 2cc00eaff0..7ebf52cfd3 100644
--- a/Library/Homebrew/formula_installer.rb
+++ b/Library/Homebrew/formula_installer.rb
@@ -20,7 +20,11 @@ class FormulaInstaller
   def self.mode_attr_accessor(*names)
     attr_accessor(*names)
     private(*names)
-    names.each { |name| define_method("#{name}?") { !!send(name) }}
+    names.each do |name|
+      predicate = "#{name}?"
+      define_method(predicate) { !!send(name) }
+      private(predicate)
+    end
   end
 
   attr_reader :formula