diff --git a/lib/plugins/conditional_request.js b/lib/plugins/conditional_request.js
index <HASH>..<HASH> 100644
--- a/lib/plugins/conditional_request.js
+++ b/lib/plugins/conditional_request.js
@@ -92,7 +92,7 @@ function checkIfNoneMatch(req, res, next) {
 
 function checkIfModified(req, res, next) {
         var ctime = req.headers['if-modified-since'];
-        var mtime = res.header('Last-Modified');
+        var mtime = res.mtime || res.header('Last-Modified');
 
         if (!mtime || !ctime)
                 return (next());
@@ -113,7 +113,7 @@ function checkIfModified(req, res, next) {
 
 function checkIfUnmodified(req, res, next) {
         var ctime = req.headers['if-unmodified-since'];
-        var mtime = res.header('Last-Modified');
+        var mtime = res.mtime || res.header('Last-Modified');
 
         if (!mtime || !ctime)
                 return (next());
Support checking res.mtime for last-modified