diff --git a/Library/Homebrew/missing_formula.rb b/Library/Homebrew/missing_formula.rb
index 8205a56634..c69de457bb 100644
--- a/Library/Homebrew/missing_formula.rb
+++ b/Library/Homebrew/missing_formula.rb
@@ -3,6 +3,9 @@
 require "formulary"
 
 module Homebrew
+  # Helper module for checking if there is a reason a formula is missing.
+  #
+  # @api private
   module MissingFormula
     class << self
       def reason(name, silent: false, show_info: false)