diff --git a/lib/hash.c b/lib/hash.c
index 6db79ea..1e097f2 100644
--- a/lib/hash.c
+++ b/lib/hash.c
@@ -31,6 +31,7 @@
 {
   struct hash *hash;
 
+  assert ((size & (size-1)) == 0);
   hash = XMALLOC (MTYPE_HASH, sizeof (struct hash));
   hash->index = XCALLOC (MTYPE_HASH_INDEX,
 			 sizeof (struct hash_backet *) * size);
@@ -71,7 +72,7 @@
   struct hash_backet *backet;
 
   key = (*hash->hash_key) (data);
-  index = key % hash->size;
+  index = key & (hash->size - 1);
 
   for (backet = hash->index[index]; backet != NULL; backet = backet->next) 
     if (backet->key == key && (*hash->hash_cmp) (backet->data, data))
@@ -125,7 +126,7 @@
   struct hash_backet *pp;
 
   key = (*hash->hash_key) (data);
-  index = key % hash->size;
+  index = key & (hash->size - 1);
 
   for (backet = pp = hash->index[index]; backet; backet = backet->next)
     {