diff --git a/lib/active_model/serializer.rb b/lib/active_model/serializer.rb
index 45a93b2b6..7724114f4 100644
--- a/lib/active_model/serializer.rb
+++ b/lib/active_model/serializer.rb
@@ -62,7 +62,7 @@ def serializer_for(resource, options = {})
         if resource.respond_to?(:serializer_class)
           resource.serializer_class
         elsif resource.respond_to?(:to_ary)
-          if Object.constants.include?(:ArraySerializer)
+          if defined?(::ArraySerializer)
             ::ArraySerializer
           else
             ArraySerializer