diff --git a/lib/rscons/varset.rb b/lib/rscons/varset.rb
index 0fa533e..429fcd3 100644
--- a/lib/rscons/varset.rb
+++ b/lib/rscons/varset.rb
@@ -44,7 +44,7 @@ module Rscons
     # @param values [VarSet, Hash] New set of variables.
     def append(values)
       values = values.vars if values.is_a?(VarSet)
-      @vars.merge!(values)
+      @vars.merge!(Marshal.load(Marshal.dump(values)))
       self
     end