diff --git a/program/lib/Roundcube/rcube_message.php b/program/lib/Roundcube/rcube_message.php
index 65949a147e..dd0efdcf9c 100644
--- a/program/lib/Roundcube/rcube_message.php
+++ b/program/lib/Roundcube/rcube_message.php
@@ -133,7 +133,7 @@ function __construct($uid, $folder = null, $is_safe = false)
         }
 
         $this->mime    = new rcube_mime($this->headers->charset);
-        $this->subject = str_replace("\n", '', $this->headers->get('subject'));
+        $this->subject = str_replace("\n", '', (string) $this->headers->get('subject'));
         $from          = $this->mime->decode_address_list($this->headers->from, 1);
         $this->sender  = current($from);