diff --git a/library/USSD_Helpers.ttcn b/library/USSD_Helpers.ttcn
index 5dd4bb1..0d23073 100644
--- a/library/USSD_Helpers.ttcn
+++ b/library/USSD_Helpers.ttcn
@@ -8,6 +8,8 @@
  *
  * Released under the terms of GNU General Public License, Version 2 or
  * (at your option) any later version.
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
  */
 
 import from General_Types all;