diff --git a/lib/active_model/serializer.rb b/lib/active_model/serializer.rb
index 54a3724d..9d779008 100644
--- a/lib/active_model/serializer.rb
+++ b/lib/active_model/serializer.rb
@@ -92,6 +92,8 @@ module ActiveModel
           serializer_class
         elsif klass.superclass
           get_serializer_for(klass.superclass)
+        else
+          nil # No serializer found
         end
       end
     end