diff --git a/Library/Homebrew/extend/os/mac/diagnostic.rb b/Library/Homebrew/extend/os/mac/diagnostic.rb
index 17b19f27fa..5c7c69e4d7 100644
--- a/Library/Homebrew/extend/os/mac/diagnostic.rb
+++ b/Library/Homebrew/extend/os/mac/diagnostic.rb
@@ -212,7 +212,7 @@ module Homebrew
         return if Homebrew::EnvConfig.developer? && OS::Mac.version.prerelease?
 
         <<~EOS
-          Ruby version #{RUBY_VERSION} is unsupported on #{MacOS.version}. Homebrew
+          Ruby version #{RUBY_VERSION} is unsupported on macOS #{MacOS.version}. Homebrew
           is developed and tested on Ruby #{required_version}, and may not work correctly
           on other Rubies. Patches are accepted as long as they don't cause breakage
           on supported Rubies.
diff --git a/Library/Homebrew/test/os/mac/diagnostic_spec.rb b/Library/Homebrew/test/os/mac/diagnostic_spec.rb
index c0a0e2573e..b51ac02193 100644
--- a/Library/Homebrew/test/os/mac/diagnostic_spec.rb
+++ b/Library/Homebrew/test/os/mac/diagnostic_spec.rb
@@ -38,7 +38,7 @@ describe Homebrew::Diagnostic::Checks do
     stub_const("RUBY_VERSION", "1.8.6")
 
     expect(checks.check_ruby_version)
-      .to match "Ruby version 1.8.6 is unsupported on 10.12"
+      .to match "Ruby version 1.8.6 is unsupported on macOS 10.12"
   end
 
   describe "#check_if_supported_sdk_available" do