diff --git a/Library/Homebrew/os/mac/mach.rbi b/Library/Homebrew/os/mac/mach.rbi
index 74fcd8ad57..0fbefdff4c 100644
--- a/Library/Homebrew/os/mac/mach.rbi
+++ b/Library/Homebrew/os/mac/mach.rbi
@@ -3,5 +3,5 @@
 module MachOShim
   include Kernel
 
-  def dirname; end
+  requires_ancestor { Pathname }
 end