diff --git a/lib/log.c b/lib/log.c
index ce00bfb..677cf9a 100644
--- a/lib/log.c
+++ b/lib/log.c
@@ -740,9 +740,9 @@
 
 /* Message lookup function. */
 const char *
-lookup (struct message *mes, int key)
+lookup (const struct message *mes, int key)
 {
-  struct message *pnt;
+  const struct message *pnt;
 
   for (pnt = mes; pnt->key != 0; pnt++) 
     if (pnt->key == key)