diff --git a/bisect_ppx.opam b/bisect_ppx.opam
index e48a8283..4e37f083 100644
--- a/bisect_ppx.opam
+++ b/bisect_ppx.opam
@@ -21,7 +21,7 @@ depends: [
   "base-unix"
   "cmdliner" {>= "1.0.0"}
   "dune" {>= "2.7.0"}
-  "ocaml" {>= "4.02.0"}
+  "ocaml" {>= "4.03.0"}
   "ppxlib" {>= "0.21.0"}
 
   "ocamlformat" {with-test & = "0.16.0"}
diff --git a/src/ppx/register.ml b/src/ppx/register.ml
index 00938fa7..e4f4d82b 100644
--- a/src/ppx/register.ml
+++ b/src/ppx/register.ml
@@ -14,7 +14,7 @@ let enabled () =
     match Sys.getenv "BISECT_ENABLE" with
     | exception Not_found ->
       `Disabled
-    | s when (String.uppercase [@ocaml.warning "-3"]) s = "YES" ->
+    | s when String.uppercase_ascii s = "YES" ->
       `Enabled
     | _ ->
       `Disabled
diff --git a/src/runtime/native/runtime.ml b/src/runtime/native/runtime.ml
index df79a51c..bb8a58c4 100644
--- a/src/runtime/native/runtime.ml
+++ b/src/runtime/native/runtime.ml
@@ -37,7 +37,7 @@ let env_to_fname env default = try Sys.getenv env with Not_found -> !default
 
 let env_to_boolean env default =
   try
-    match (String.uppercase [@ocaml.warning "-3"]) (Sys.getenv env) with
+    match String.uppercase_ascii (Sys.getenv env) with
     | "YES" -> true
     | "NO" -> false
     | _ -> default
@@ -46,7 +46,7 @@ let env_to_boolean env default =
 let verbose =
   lazy begin
     let fname = env_to_fname "BISECT_SILENT" default_bisect_silent in
-    match (String.uppercase [@ocaml.warning "-3"]) fname with
+    match String.uppercase_ascii fname with
     | "YES" | "ON" -> fun _ -> ()
     | "ERR"        -> fun msg -> prerr_endline (string_of_message msg)
     | _uc_fname    ->