diff --git a/pylti1p3/message_launch.py b/pylti1p3/message_launch.py
index 4143900..6f6651d 100644
--- a/pylti1p3/message_launch.py
+++ b/pylti1p3/message_launch.py
@@ -775,7 +775,7 @@ def set_launch_data_storage(
             if session_id:
                 data_storage.set_session_id(session_id)
             else:
-                raise LtiException(f"Missing %s cookie {session_cookie_name}")
+                raise LtiException(f"Missing cookie: {session_cookie_name}")
         self._session_service.set_data_storage(data_storage)
         return self