diff --git a/Library/Homebrew/test/os/mac/diagnostic_spec.rb b/Library/Homebrew/test/os/mac/diagnostic_spec.rb
index b2183082ae..36e7f6f2b9 100644
--- a/Library/Homebrew/test/os/mac/diagnostic_spec.rb
+++ b/Library/Homebrew/test/os/mac/diagnostic_spec.rb
@@ -35,6 +35,7 @@ describe Homebrew::Diagnostic::Checks do
     allow(OS::Mac).to receive(:version).and_return(macos_version)
     allow(OS::Mac).to receive(:full_version).and_return(macos_version)
     stub_const("RUBY_VERSION", "1.8.6")
+    ENV["HOMEBREW_MACOS_SYSTEM_RUBY_NEW_ENOUGH"] = "1"
 
     expect(checks.check_ruby_version)
       .to match "Ruby version 1.8.6 is unsupported on macOS 10.12"