diff --git a/isisd/isis_pdu.c b/isisd/isis_pdu.c
index 4311a90..a2ab064 100644
--- a/isisd/isis_pdu.c
+++ b/isisd/isis_pdu.c
@@ -64,7 +64,7 @@
 #endif /* PNBBY */
 
 /* Utility mask array. */
-static u_char maskbit[] = {
+static const u_char maskbit[] = {
   0x00, 0x80, 0xc0, 0xe0, 0xf0, 0xf8, 0xfc, 0xfe, 0xff
 };