diff --git a/Library/Homebrew/os/mac/mach.rbi b/Library/Homebrew/os/mac/mach.rbi
index 54bb844d78..74fcd8ad57 100644
--- a/Library/Homebrew/os/mac/mach.rbi
+++ b/Library/Homebrew/os/mac/mach.rbi
@@ -2,4 +2,6 @@
 
 module MachOShim
   include Kernel
+
+  def dirname; end
 end