diff --git a/src/option.c b/src/option.c
index 3052a4d..9ffd7fc 100644
--- a/src/option.c
+++ b/src/option.c
@@ -3322,7 +3322,7 @@ static int one_opt(int option, char *arg, char *errstr, char *gen_err, int comma
 			
 			if (!atoi_check(pref, &new_addr->prefixlen) ||
 			    new_addr->prefixlen > 128 ||
-			    (((1<<(128-new_addr->prefixlen))-1) & addrpart) != 0)
+			    ((((u64)1<<(128-new_addr->prefixlen))-1) & addrpart) != 0)
 			  {
 			    dhcp_config_free(new);
 			    ret_err(_("bad IPv6 prefix"));
diff --git a/src/rfc3315.c b/src/rfc3315.c
index 57f3f8b..b3f0a0a 100644
--- a/src/rfc3315.c
+++ b/src/rfc3315.c
@@ -1731,7 +1731,7 @@ static int config_valid(struct dhcp_config *config, struct dhcp_context *context
 	addresses = 1;
 	
 	if (addr_list->flags & ADDRLIST_PREFIX)
-	  addresses = 1<<(128-addr_list->prefixlen);
+	  addresses = (u64)1<<(128-addr_list->prefixlen);
 	
 	if ((addr_list->flags & ADDRLIST_WILDCARD))
 	  {