diff --git a/lib/rack/session/abstract/id.rb b/lib/rack/session/abstract/id.rb
index b15ee3b81..74ee2e8e4 100644
--- a/lib/rack/session/abstract/id.rb
+++ b/lib/rack/session/abstract/id.rb
@@ -182,7 +182,7 @@ def load!
         end
 
         def stringify_keys(other)
-          other.transform_keys(&:to_s)
+          other.to_hash.transform_keys(&:to_s)
         end
       end