diff --git a/include/libtorrent/alert_types.hpp b/include/libtorrent/alert_types.hpp
index bbeb94ed9..271593a5c 100644
--- a/include/libtorrent/alert_types.hpp
+++ b/include/libtorrent/alert_types.hpp
@@ -1390,7 +1390,9 @@ namespace libtorrent
 	// This alert is generated to log informational events related to either
 	// UPnP or NAT-PMP. They contain a log line and the type (0 = NAT-PMP
 	// and 1 = UPnP). Displaying these messages to an end user is only useful
-	// for debugging the UPnP or NAT-PMP implementation.
+	// for debugging the UPnP or NAT-PMP implementation. This alert is only
+	// posted if the alert::port_mapping_log_notification flag is enabled in
+	// the alert mask.
 	struct TORRENT_EXPORT portmap_log_alert: alert
 	{
 		// internal