diff --git a/lib/redmine_dmsf/webdav/controller.rb b/lib/redmine_dmsf/webdav/controller.rb
index cced9480..bd33f616 100644
--- a/lib/redmine_dmsf/webdav/controller.rb
+++ b/lib/redmine_dmsf/webdav/controller.rb
@@ -95,7 +95,7 @@ module RedmineDmsf
         end
         response.body = doc.to_xml
         response["Content-Type"] = 'application/xml; charset="utf-8"'
-        response["Content-Length"] = response.body.size.to_s
+        response["Content-Length"] = response.body.bytesize.to_s
       end