diff --git a/middleman-core/lib/middleman-core/meta_pages/sitemap_resource.rb b/middleman-core/lib/middleman-core/meta_pages/sitemap_resource.rb
index b345c1c0..2b086eae 100644
--- a/middleman-core/lib/middleman-core/meta_pages/sitemap_resource.rb
+++ b/middleman-core/lib/middleman-core/meta_pages/sitemap_resource.rb
@@ -18,10 +18,10 @@ module Middleman
         classes << 'ignored' if @resource.ignored?
         content_tag :div, class: classes.join(' ') do
           content_tag :table do
-            content = ''.dup
+            content = +''
             resource_properties.each do |label, value|
               content << content_tag(:tr) do
-                row_content = ''.dup
+                row_content = +''
                 row_content << content_tag(:th, label)
                 row_content << content_tag(:td, value)
                 row_content.html_safe