diff --git a/tool/rbinstall.rb b/tool/rbinstall.rb
index a5468f0d0f..92e54c6774 100755
--- a/tool/rbinstall.rb
+++ b/tool/rbinstall.rb
@@ -306,7 +306,7 @@ goruby_install_name = "go" + ruby_install_name
 bindir = CONFIG["bindir", true]
 libdir = CONFIG["libdir", true]
 rubyhdrdir = CONFIG["rubyhdrdir", true]
-archhdrdir = CONFIG["rubyarchhdrdir"] || (rubyhdrdir + "/" + $arch)
+archhdrdir = CONFIG["rubyarchhdrdir"] || (rubyhdrdir + "/" + CONFIG['arch'])
 rubylibdir = CONFIG["rubylibdir", true]
 archlibdir = CONFIG["rubyarchdir", true]
 sitelibdir = CONFIG["sitelibdir"]