diff --git a/src/log.h b/src/log.h
index 8564db1..368a3ef 100644
--- a/src/log.h
+++ b/src/log.h
@@ -76,6 +76,8 @@
  *
  * This callback is set for libnetconf2 and also libyang that is used internally. libyang
  * callback can be set explicitly, but must be done so after calling this function.
+ * The callback is not set per-session, it is a global resource. It might be called with
+ * a NULL session parameter.
  *
  * @param[in] clb Callback that is called for every message.
  */