diff --git a/lib/active_model/serializer.rb b/lib/active_model/serializer.rb
index 9f62e6157..418f10bae 100644
--- a/lib/active_model/serializer.rb
+++ b/lib/active_model/serializer.rb
@@ -169,7 +169,7 @@ def initialize(object, options = {})
     end
 
     def json_key
-      @root || self.class.root_name
+      @root || object.class.model_name.to_s.downcase
     end
 
     def id