diff --git a/src/logging.c b/src/logging.c
index 7102543..2a8bfdc 100644
--- a/src/logging.c
+++ b/src/logging.c
@@ -339,9 +339,11 @@
 	if (subsys < 0)
 		subsys = subsys_lib2index(subsys);
 
-	if (subsys > osmo_log_info->num_cat)
+	if (subsys < 0 || subsys >= osmo_log_info->num_cat)
 		subsys = subsys_lib2index(DLGLOBAL);
 
+	OSMO_ASSERT(!(subsys < 0 || subsys >= osmo_log_info->num_cat));
+
 	return subsys;
 }