diff --git a/Library/Homebrew/extend/os/mac/diagnostic.rb b/Library/Homebrew/extend/os/mac/diagnostic.rb
index ae16cf5624..53cf3cd724 100644
--- a/Library/Homebrew/extend/os/mac/diagnostic.rb
+++ b/Library/Homebrew/extend/os/mac/diagnostic.rb
@@ -51,7 +51,7 @@ module Homebrew
         if OS::Mac.prerelease?
           what = "pre-release version"
         elsif OS::Mac.outdated_release?
-          who << " (and Apple)"
+          +who << " (and Apple)"
           what = "old version"
         else
           return