diff --git a/ospf6d/ospf6_proto.c b/ospf6d/ospf6_proto.c
index 7ee7e0a..5b47cf8 100644
--- a/ospf6d/ospf6_proto.c
+++ b/ospf6d/ospf6_proto.c
@@ -36,12 +36,15 @@
   offset = op->prefix_length % 8;
   mask = 0xff << (8 - offset);
 
-  if (index >= 16)
+  if (index > 16)
     {
-      zlog_warn ("Apply mask to ospf6_prefix failed");
+      zlog_warn ("Prefix length too long: %d", op->prefix_length);
       return;
     }
 
+  if (index == 16)
+    return;
+
   pnt[index] &= mask;
   index ++;