diff --git a/Library/Homebrew/cask/cache.rb b/Library/Homebrew/cask/cache.rb
index 1cd57f9060..911a4eb833 100644
--- a/Library/Homebrew/cask/cache.rb
+++ b/Library/Homebrew/cask/cache.rb
@@ -6,9 +6,10 @@ module Cask
   #
   # @api private
   module Cache
-    module_function
+    extend T::Sig
 
-    def path
+    sig { returns(Pathname) }
+    def self.path
       @path ||= HOMEBREW_CACHE/"Cask"
     end
   end