diff --git a/program/lib/Roundcube/rcube_smtp.php b/program/lib/Roundcube/rcube_smtp.php
index 6c0f3cb04..143711163 100644
--- a/program/lib/Roundcube/rcube_smtp.php
+++ b/program/lib/Roundcube/rcube_smtp.php
@@ -172,8 +172,8 @@ class rcube_smtp
             return false;
         }
 
-        $smtp_user = str_replace('%u', $rcube->get_user_name(), $CONFIG['smtp_user']);
-        $smtp_pass = str_replace('%p', $rcube->get_user_password(), $CONFIG['smtp_pass']);
+        $smtp_user = str_replace('%u', (string) $rcube->get_user_name(), $CONFIG['smtp_user']);
+        $smtp_pass = str_replace('%p', (string) $rcube->get_user_password(), $CONFIG['smtp_pass']);
         $smtp_auth_type = $CONFIG['smtp_auth_type'] ?: null;
         $smtp_authz     = null;