diff --git a/src/alert.cpp b/src/alert.cpp
index cf303a128..7892c7646 100644
--- a/src/alert.cpp
+++ b/src/alert.cpp
@@ -178,7 +178,7 @@ namespace libtorrent {
 	std::string tracker_announce_alert::message() const
 	{
 		const static char* event_str[] = {"none", "completed", "started", "stopped", "paused"};
-		TORRENT_ASSERT_VAL(event < sizeof(event_str)/sizeof(event_str), event);
+		TORRENT_ASSERT_VAL(event < sizeof(event_str)/sizeof(event_str[0]), event);
 		return tracker_alert::message() + " sending announce (" + event_str[event] + ")";
 	}