diff --git a/Library/Homebrew/extend/os/mac/diagnostic.rb b/Library/Homebrew/extend/os/mac/diagnostic.rb
index ae16cf5624..696c48db04 100644
--- a/Library/Homebrew/extend/os/mac/diagnostic.rb
+++ b/Library/Homebrew/extend/os/mac/diagnostic.rb
@@ -47,7 +47,7 @@ module Homebrew
       def check_for_unsupported_macos
         return if ARGV.homebrew_developer?
 
-        who = "We"
+        who = +"We"
         if OS::Mac.prerelease?
           what = "pre-release version"
         elsif OS::Mac.outdated_release?
@@ -56,6 +56,7 @@ module Homebrew
         else
           return
         end
+        who.freeze
 
         <<~EOS
           You are using macOS #{MacOS.version}.