diff --git a/Library/Homebrew/system_config.rb b/Library/Homebrew/system_config.rb
index d70a43534c..a17b57fb23 100644
--- a/Library/Homebrew/system_config.rb
+++ b/Library/Homebrew/system_config.rb
@@ -139,6 +139,7 @@ class SystemConfig
         HOMEBREW_GIT_CONFIG_FILE
         HOMEBREW_LIBRARY
         HOMEBREW_MACOS_VERSION
+        HOMEBREW_MACOS_VERSION_NUMERIC
         HOMEBREW_RUBY_PATH
         HOMEBREW_SYSTEM
         HOMEBREW_SYSTEM_TEMP