diff --git a/lib/action_controller/serialization.rb b/lib/action_controller/serialization.rb
index d8aaef9a..1593dd14 100644
--- a/lib/action_controller/serialization.rb
+++ b/lib/action_controller/serialization.rb
@@ -60,7 +60,15 @@ module ActionController
     private
 
     def namespace_for_serializer
-      @namespace_for_serializer ||= self.class.parent unless self.class.parent == Object
+      @namespace_for_serializer ||= namespace_for_class(self.class) unless namespace_for_class(self.class) == Object
+    end
+
+    def namespace_for_class(klass)
+      if Module.method_defined?(:module_parent)
+        klass.module_parent
+      else
+        klass.parent
+      end
     end
 
     def default_serializer(resource)