diff --git a/actionpack/lib/action_dispatch/middleware/session/cookie_store.rb b/actionpack/lib/action_dispatch/middleware/session/cookie_store.rb
index 8ebf870b95f8ba0f673d965dcd60ff8020150cf7..a4866f5a8facb07ee90634c968057df2d0517564 100644
--- a/actionpack/lib/action_dispatch/middleware/session/cookie_store.rb
+++ b/actionpack/lib/action_dispatch/middleware/session/cookie_store.rb
@@ -59,7 +59,8 @@ def unpacked_cookie_data(env)
       end
 
       def set_session(env, sid, session_data, options)
-        session_data.merge("session_id" => sid)
+        session_data["session_id"] = sid
+        session_data
       end
 
       def set_cookie(env, session_id, cookie)
diff --git a/actionpack/test/template/form_helper_test.rb b/actionpack/test/template/form_helper_test.rb
index 3b1af47770b6a9888b9967d47103d7d7155e3909..c5a32635f86fd41ff68028af94ac77b8631207b9 100644
--- a/actionpack/test/template/form_helper_test.rb
+++ b/actionpack/test/template/form_helper_test.rb
@@ -2238,7 +2238,7 @@ def initialize(object_name, object, template, options, block)
       end
     end
 
-    assert_deprecated /Giving a block to FormBuilder is deprecated and has no effect anymore/ do
+    assert_deprecated(/Giving a block to FormBuilder is deprecated and has no effect anymore/) do
       builder_class.new(:foo, nil, nil, {}, proc {})
     end
   end