diff --git a/rng/rng.ml b/rng/rng.ml
index 475a8f69..52603ec8 100644
--- a/rng/rng.ml
+++ b/rng/rng.ml
@@ -73,6 +73,9 @@ let get = function Some g -> g | None -> default_generator ()
 let generate_into ?(g = default_generator ()) b ?(off = 0) n =
   let Generator (g, _, m) = g in
   let module M = (val m) in
+  if off < 0 || n < 0 then
+    invalid_arg ("negative offset " ^ string_of_int off ^ " or length " ^
+                 string_of_int n);
   if Bytes.length b - off < n then
     invalid_arg "buffer too short";
   M.generate_into ~g b ~off n