diff --git a/tools/ocaml/libs/xb/xs_ring.ml b/tools/ocaml/libs/xb/xs_ring.ml
index db7f86bd27..dd5e014a33 100644
--- a/tools/ocaml/libs/xb/xs_ring.ml
+++ b/tools/ocaml/libs/xb/xs_ring.ml
@@ -25,14 +25,11 @@ module Server_features = Set.Make(struct
 end)
 
 external read: Xenmmap.mmap_interface -> bytes -> int -> int = "ml_interface_read"
-external write: Xenmmap.mmap_interface -> bytes -> int -> int = "ml_interface_write"
+external write_substring: Xenmmap.mmap_interface -> string -> int -> int = "ml_interface_write"
 
 external _internal_set_server_features: Xenmmap.mmap_interface -> int -> unit = "ml_interface_set_server_features" [@@noalloc]
 external _internal_get_server_features: Xenmmap.mmap_interface -> int = "ml_interface_get_server_features" [@@noalloc]
 
-let write_substring mmap buff len =
-	write mmap (Bytes.unsafe_of_string buff) len
-
 let get_server_features mmap =
 	(* NB only one feature currently defined above *)
 	let x = _internal_get_server_features mmap in
diff --git a/tools/ocaml/libs/xb/xs_ring_stubs.c b/tools/ocaml/libs/xb/xs_ring_stubs.c
index 1f58524535..1243c63f03 100644
--- a/tools/ocaml/libs/xb/xs_ring_stubs.c
+++ b/tools/ocaml/libs/xb/xs_ring_stubs.c
@@ -112,7 +112,7 @@ CAMLprim value ml_interface_write(value ml_interface,
 	CAMLlocal1(ml_result);
 
 	struct mmap_interface *interface = GET_C_STRUCT(ml_interface);
-	const unsigned char *buffer = Bytes_val(ml_buffer);
+	const char *buffer = String_val(ml_buffer);
 	int len = Int_val(ml_len);
 	int result;