diff --git a/lib/webrat/merb.rb b/lib/webrat/merb.rb
index 35258e9..4f2b579 100644
--- a/lib/webrat/merb.rb
+++ b/lib/webrat/merb.rb
@@ -5,7 +5,7 @@ gem "extlib"
 require "extlib"
 require "merb-core"
 
-HashWithIndifferentAccess = Mash
+HashWithIndifferentAccess = Mash unless defined?(HashWithIndifferentAccess)
 
 module Webrat
   class MerbSession < Session #:nodoc: