diff --git a/csrf.php b/csrf.php
index bb34ea6..f19de8f 100644
--- a/csrf.php
+++ b/csrf.php
@@ -110,7 +110,8 @@ class csrf
     if (! isset ($_SESSION["domframework"]["csrf"]))
     {
       throw new \Exception (dgettext ("domframework",
-        "No previous CSRF token : abort"), 406);
+        "No previous CSRF token found in session ".
+        "(maybe a new session after expiration ?) : abort"), 406);
     }
     if (! key_exists ($tokenFromUser, $_SESSION["domframework"]["csrf"]))
     {