diff --git a/lib/active_model/serializer/adapter.rb b/lib/active_model/serializer/adapter.rb
index 5d33650e..9c95f5ee 100644
--- a/lib/active_model/serializer/adapter.rb
+++ b/lib/active_model/serializer/adapter.rb
@@ -83,7 +83,7 @@ module ActiveModel
       end
 
       def root
-        serializer.json_key.to_sym
+        serializer.json_key.to_sym if serializer.json_key
       end
 
       def include_meta(json)