No warnings here any more.
diff --git a/isisd/isis_events.c b/isisd/isis_events.c
index 353c29b..1193b26 100644
--- a/isisd/isis_events.c
+++ b/isisd/isis_events.c
@@ -317,7 +317,7 @@
 }
 
 void
-isis_event_auth_failure (char *area_tag, char *error_string, u_char *sysid)
+isis_event_auth_failure (char *area_tag, const char *error_string, u_char *sysid)
 {
   zlog_info ("ISIS-Evt (%s) Authentication failure %s from %s",
 	     area_tag, error_string, sysid_print (sysid));