diff --git a/middleman-core/lib/middleman-core/sitemap/resource.rb b/middleman-core/lib/middleman-core/sitemap/resource.rb
index 7c18078f..ce1bc100 100644
--- a/middleman-core/lib/middleman-core/sitemap/resource.rb
+++ b/middleman-core/lib/middleman-core/sitemap/resource.rb
@@ -85,7 +85,7 @@ module Middleman
         file_descriptor && file_descriptor[:full_path].to_s
       end
 
-      Contract Or[Symbol, String]
+      Contract Or[Symbol, String, Fixnum]
       def page_id
         metadata[:page][:id] || destination_path
       end