diff --git a/drivers/ocaml64.drv b/drivers/ocaml64.drv
index d83f52fde35ebadba923703b78be3e580be014cd..61b8a0d87d14b6ee950fe3a416f2b077a1512c8e 100644
--- a/drivers/ocaml64.drv
+++ b/drivers/ocaml64.drv
@@ -53,7 +53,7 @@ theory list.Mem
 end
 
 theory list.Nth
-  syntax function nth "List.nth %1 %2"
+  syntax function nth "List.nth_opt %2 (Z.to_int %1)"
 end
 
 theory list.Append