diff --git a/lib/tracking_number/active_model_validator.rb b/lib/tracking_number/active_model_validator.rb
index 62376f7..190402a 100644
--- a/lib/tracking_number/active_model_validator.rb
+++ b/lib/tracking_number/active_model_validator.rb
@@ -6,7 +6,7 @@ def validate_each(record, attribute, value)
     elsif TrackingNumber.new(value).valid?
       # looks good to me
     else
-      record.errors[attribute] << options[:message] || 'is not a valid tracking number'
+      record.errors[attribute] << (options[:message] || 'is not a valid tracking number')
     end
   end
 end