diff --git a/evthread.c b/evthread.c
index d9c54787..a8b2534f 100644
--- a/evthread.c
+++ b/evthread.c
@@ -156,6 +156,7 @@ debug_lock_free(void *lock_, unsigned locktype)
 	}
 	lock->lock = NULL;
 	lock->count = -100;
+	lock->signature = 0x12300fda;
 	mm_free(lock);
 }