diff --git a/compiler/gen_OCaml.ml b/compiler/gen_OCaml.ml
index b0b245f..d70b0c6 100644
--- a/compiler/gen_OCaml.ml
+++ b/compiler/gen_OCaml.ml
@@ -364,9 +364,7 @@ let rec default_value (ev_regime : Gencode.ev_regime) t =
     | Bitstring32 _ -> None
     | Sum (l, _) -> begin (* first constant constructor = default*)
         match
-          try
-            Some (List.find_map (function `Constant x -> Some x | _ -> None) l)
-          with Not_found -> None
+          List.find_map_opt (function `Constant x -> Some x | _ -> None) l
         with
           | None -> None
           | Some c ->
diff --git a/opam b/opam
index 7adc9cf..67a7e70 100644
--- a/opam
+++ b/opam
@@ -24,6 +24,6 @@ depends: [
   "ocamlfind" {build}
   "ounit" {test}
   "camlp4" {build}
-  ("extlib" | "extlib-compat")
+  ("extlib" {>= "1.7.7"} | "extlib-compat" {>="1.7.7"})
   "base-bytes"
 ]
diff --git a/runtime/error.ml b/runtime/error.ml
index 84f143c..8d7d3f8 100644
--- a/runtime/error.ml
+++ b/runtime/error.ml
@@ -28,9 +28,9 @@ let rec pp_location pp = function
   | Field (field, loc) ->
       PP.fprintf pp "@[<1>%s.@,%a@]" field pp_location loc
   | Message (msg, constr, loc) -> match constr with
-        None -> PP.fprintf pp "@[<1>%s.@,%a@]" (String.capitalize msg) pp_location loc
+        None -> PP.fprintf pp "@[<1>%s.@,%a@]" (String.capitalize_ascii msg) pp_location loc
       | Some c -> PP.fprintf pp "@[<1>%s_%s.@,%a@]"
-                    (String.capitalize msg) (String.capitalize c) pp_location loc
+                    (String.capitalize_ascii msg) (String.capitalize_ascii c) pp_location loc
 
 let pp_format_error pp = function
   | Bad_wire_type ty ->
diff --git a/runtime/msg_buffer.ml b/runtime/msg_buffer.ml
index c71bd12..397e986 100644
--- a/runtime/msg_buffer.ml
+++ b/runtime/msg_buffer.ml
@@ -108,7 +108,7 @@ let resize b more =
 let add_char b c =
   let pos = b.position in
   if pos >= b.length then resize b 1;
-  b.buffer.[pos] <- c;
+  Bytes.set b.buffer pos c;
   b.position <- pos + 1
 
 let add_substring b s offset len =
diff --git a/runtime/random_gen.ml b/runtime/random_gen.ml
index fd69e12..6222a23 100644
--- a/runtime/random_gen.ml
+++ b/runtime/random_gen.ml
@@ -67,7 +67,7 @@ struct
       let s = Bytes.create n in
       let rec loop = function
           n when n < 0 -> return @@ Bytes.unsafe_to_string s
-        | n -> rand_integer 255 >>= fun c -> s.[n] <- Char.chr c; loop (n - 1)
+        | n -> rand_integer 255 >>= fun c -> Bytes.set s n (Char.chr c); loop (n - 1)
       in loop (n - 1)
 
   let rand_readable_string len =
@@ -75,7 +75,7 @@ struct
       let s = Bytes.create n in
       let rec loop = function
           n when n < 0 -> return @@ Bytes.unsafe_to_string s
-        | n -> rand_integer (127 - 32) >>= fun c -> s.[n] <- Char.chr (32 + c); loop (n - 1)
+        | n -> rand_integer (127 - 32) >>= fun c -> Bytes.set s n (Char.chr (32 + c)); loop (n - 1)
       in loop (n - 1)
 
   let rand_int =