diff --git a/lib/prefix.c b/lib/prefix.c
index 7dc866d..61a278c 100644
--- a/lib/prefix.c
+++ b/lib/prefix.c
@@ -70,15 +70,16 @@
 {
   int offset;
   int shift;
-
-  /* Set both prefix's head pointer. */
-  const u_char *np = (const u_char *)&n->u.prefix;
-  const u_char *pp = (const u_char *)&p->u.prefix;
+  const u_char *np, *pp;
 
   /* If n's prefix is longer than p's one return 0. */
   if (n->prefixlen > p->prefixlen)
     return 0;
 
+  /* Set both prefix's head pointer. */
+  np = (const u_char *)&n->u.prefix;
+  pp = (const u_char *)&p->u.prefix;
+  
   offset = n->prefixlen / PNBBY;
   shift =  n->prefixlen % PNBBY;