diff --git a/Library/Homebrew/system_config.rb b/Library/Homebrew/system_config.rb
index d6e2f47111..341be88fde 100644
--- a/Library/Homebrew/system_config.rb
+++ b/Library/Homebrew/system_config.rb
@@ -183,6 +183,7 @@ class SystemConfig
         HOMEBREW_LIBRARY
         HOMEBREW_MACOS_VERSION
         HOMEBREW_RUBY_PATH
+        HOMEBREW_RUBY_WARNINGS
         HOMEBREW_SYSTEM
         HOMEBREW_OS_VERSION
         HOMEBREW_PATH