diff --git a/pppd/plugins/radius/clientid.c b/pppd/plugins/radius/clientid.c
index d49579c..7de0211 100644
--- a/pppd/plugins/radius/clientid.c
+++ b/pppd/plugins/radius/clientid.c
@@ -110,7 +110,7 @@ UINT4 rc_map2id(char *name)
 	if (*name != '/')
 		strcpy(ttyname, "/dev/");
 
-	strncat(ttyname, name, sizeof(ttyname));
+	strncat(ttyname, name, sizeof(ttyname)-strlen(ttyname)-1);
 
 	for(p = map2id_list; p; p = p->next)
 		if (!strcmp(ttyname, p->name)) return p->id;