diff --git a/src/Symfony/Component/HttpKernel/HttpCache/HttpCache.php b/src/Symfony/Component/HttpKernel/HttpCache/HttpCache.php
index addeca8bae143..ef52e350185d3 100644
--- a/src/Symfony/Component/HttpKernel/HttpCache/HttpCache.php
+++ b/src/Symfony/Component/HttpKernel/HttpCache/HttpCache.php
@@ -352,7 +352,7 @@ protected function validate(Request $request, Response $entry, $catch = false)
         }
 
         // add our cached last-modified validator
-        $subRequest->headers->set('if_modified_since', $entry->headers->get('Last-Modified'));
+        $subRequest->headers->set('if_modified_since', $entry->headers->get('Last-Modified', ''));
 
         // Add our cached etag validator to the environment.
         // We keep the etags from the client to handle the case when the client