diff --git a/lib/stripe/stripe_object.rb b/lib/stripe/stripe_object.rb
index 9ba927db5..3aca0757e 100644
--- a/lib/stripe/stripe_object.rb
+++ b/lib/stripe/stripe_object.rb
@@ -64,7 +64,8 @@ def self.additive_object_param(name)
     # Returns whether the given name is an additive object parameter. See
     # `.additive_object_param` for details.
     def self.additive_object_param?(name)
-      !@additive_params.nil? && @additive_params.include?(name)
+      @additive_params ||= Set.new
+      @additive_params.include?(name)
     end
 
     def initialize(id = nil, opts = {})