diff --git a/lib/prefix.c b/lib/prefix.c
index 2afaa09..c85e659 100644
--- a/lib/prefix.c
+++ b/lib/prefix.c
@@ -29,7 +29,7 @@
 #include "log.h"
 
 /* Maskbit. */
-static u_char maskbit[] = {0x00, 0x80, 0xc0, 0xe0, 0xf0,
+static const u_char maskbit[] = {0x00, 0x80, 0xc0, 0xe0, 0xf0,
 			         0xf8, 0xfc, 0xfe, 0xff};
 
 /* Number of bits in prefix type. */