diff --git a/pandoc/sitemap.lua b/pandoc/sitemap.lua
index 1870988..40f746a 100644
--- a/pandoc/sitemap.lua
+++ b/pandoc/sitemap.lua
@@ -150,13 +150,12 @@ table.insert(lines, "</article>")
 local data = table.concat(lines, "\n")
 
 -- read existing data from output_file
-local f = assert(io.open(output_file), "\n" ..
-  "[ERROR] could not open " .. output_file .. " for reading.")
-local data_old = f:read("a")
-f:close()
+local f = io.open(output_file)
+local data_old
+if f then data_old = f:read("a"); f:close() end
 
 -- write to output_file if data changed
-if data ~= data_old then
+if not data_old or data ~= data_old then
   local f = assert(io.open(output_file, "w"), "\n" ..
     "[ERROR] could not open " .. output_file .. " for writing.")
   f:write(data)