diff --git a/tool/mkconfig.rb b/tool/mkconfig.rb
index 49b841f..c0e230d 100755
--- a/tool/mkconfig.rb
+++ b/tool/mkconfig.rb
@@ -267,13 +267,11 @@ def RbConfig::expand(val, config = CONFIG)
 	var
       end
     }
-    val.replace(newval) unless newval == val
-    val
-  end
-  CONFIG.each_value do |val|
-    RbConfig::expand(val)
+    newval != val ? newval : val
   end
 
+  CONFIG.update(CONFIG) { |_, val| RbConfig::expand(val) }
+
   # returns the absolute pathname of the ruby command.
   def RbConfig.ruby
     File.join(