diff --git a/ospf6d/ospf6_spf.c b/ospf6d/ospf6_spf.c
index 79539b2..4f3af7c 100644
--- a/ospf6d/ospf6_spf.c
+++ b/ospf6d/ospf6_spf.c
@@ -527,7 +527,7 @@
 }
 
 void
-ospf6_spf_display_subtree (struct vty *vty, char *prefix, int rest,
+ospf6_spf_display_subtree (struct vty *vty, const char *prefix, int rest,
                            struct ospf6_vertex *v)
 {
   struct listnode *node;