diff --git a/Library/Homebrew/extend/os/mac/diagnostic.rb b/Library/Homebrew/extend/os/mac/diagnostic.rb
index 58d8c633d4..913a5762b3 100644
--- a/Library/Homebrew/extend/os/mac/diagnostic.rb
+++ b/Library/Homebrew/extend/os/mac/diagnostic.rb
@@ -195,7 +195,8 @@ module Homebrew
       end
 
       def check_ruby_version
-        return if RUBY_VERSION[/\d\.\d/] == "2.0"
+        ruby_version = "2.0"
+        return if RUBY_VERSION[/\d\.\d/] == ruby_version
 
         <<-EOS.undent
           Ruby version #{RUBY_VERSION} is unsupported on #{MacOS.version}. Homebrew
diff --git a/Library/Homebrew/test/os/mac/diagnostic_spec.rb b/Library/Homebrew/test/os/mac/diagnostic_spec.rb
index 787f80fece..19682e0c78 100644
--- a/Library/Homebrew/test/os/mac/diagnostic_spec.rb
+++ b/Library/Homebrew/test/os/mac/diagnostic_spec.rb
@@ -45,4 +45,18 @@ describe Homebrew::Diagnostic::Checks do
     expect(subject.check_homebrew_prefix)
       .to match("Your Homebrew's prefix is not /usr/local.")
   end
+
+  specify "#check_ruby_version" do
+    expected_string = <<-EXPECTED
+Ruby version 2.3.3p222 is unsupported on 10.13. Homebrew
+is developed and tested on Ruby 2.0, and may not work correctly
+on other Rubies. Patches are accepted as long as they don't cause breakage
+on supported Rubies.
+    EXPECTED
+    allow(MacOS).to receive(:version).and_return(OS::Mac::Version.new("10.13"))
+    stub_const("RUBY_VERSION", "2.3.3p222")
+
+    expect(subject.check_ruby_version)
+      .to match(expected_string)
+  end
 end