diff --git a/ospf6d/ospf6_spf.h b/ospf6d/ospf6_spf.h
index 6a6424e..c9e031b 100644
--- a/ospf6d/ospf6_spf.h
+++ b/ospf6d/ospf6_spf.h
@@ -82,7 +82,7 @@
                             struct ospf6_area *oa);
 void ospf6_spf_schedule (struct ospf6_area *oa);
 
-void ospf6_spf_display_subtree (struct vty *vty, char *prefix,
+void ospf6_spf_display_subtree (struct vty *vty, const char *prefix,
                                 int rest, struct ospf6_vertex *v);
 
 int config_write_ospf6_debug_spf (struct vty *vty);