diff --git a/actionpack/lib/action_dispatch/request/session.rb b/actionpack/lib/action_dispatch/request/session.rb
index <HASH>..<HASH> 100644
--- a/actionpack/lib/action_dispatch/request/session.rb
+++ b/actionpack/lib/action_dispatch/request/session.rb
@@ -37,7 +37,7 @@ module ActionDispatch
         def initialize(by, env, default_options)
           @by       = by
           @env      = env
-          @delegate = default_options
+          @delegate = default_options.dup
         end
 
         def [](key)
need to dup the default options so that mutations will not impact us