diff --git a/src/thread_local_storage.real.ml b/src/thread_local_storage.real.ml
index 70d7a558..768e2c35 100644
--- a/src/thread_local_storage.real.ml
+++ b/src/thread_local_storage.real.ml
@@ -5,8 +5,13 @@ module A = Atomic_
 (* sanity check *)
 let () = assert (Obj.field (Obj.repr (Thread.self ())) 1 = Obj.repr ())
 
+type view = ..
+type view += Sentinel
+
 type 'a key = {
   index: int;  (** Unique index for this key. *)
+  unwrap: view -> 'a option;
+  wrap: 'a -> view;
   compute: unit -> 'a;
       (** Initializer for values for this key. Called at most
         once per thread. *)
@@ -15,12 +20,17 @@ type 'a key = {
 (** Counter used to allocate new keys *)
 let counter = A.make 0
 
-(** Value used to detect a TLS slot that was not initialized yet *)
-let[@inline] sentinel_value_for_uninit_tls_ () : Obj.t = Obj.repr counter
-
-let new_key compute : _ key =
+let new_key (type a) compute : a key =
+  let module M = struct
+    type view += V of a
+  end in
   let index = A.fetch_and_add counter 1 in
-  { index; compute }
+  let wrap x = M.V x in
+  let unwrap = function
+    | M.V x -> Some x
+    | _ -> None
+  in
+  { index; compute; wrap; unwrap }
 
 type thread_internal_state = {
   _id: int;  (** Thread ID (here for padding reasons) *)
@@ -42,13 +52,13 @@ let ceil_pow_2_minus_1 (n : int) : int =
     n
 
 (** Grow the array so that [index] is valid. *)
-let[@inline never] grow_tls (old : Obj.t array) (index : int) : Obj.t array =
+let[@inline never] grow_tls (old : view array) (index : int) : view array =
   let new_length = ceil_pow_2_minus_1 (index + 1) in
-  let new_ = Array.make new_length (sentinel_value_for_uninit_tls_ ()) in
+  let new_ = Array.make new_length Sentinel in
   Array.blit old 0 new_ 0 (Array.length old);
   new_
 
-let[@inline] get_tls_ (index : int) : Obj.t array =
+let[@inline] get_tls_ (index : int) : view array =
   let thread : thread_internal_state = Obj.magic (Thread.self ()) in
   let tls = thread.tls in
   if Obj.is_int tls then (
@@ -56,7 +66,7 @@ let[@inline] get_tls_ (index : int) : Obj.t array =
     thread.tls <- Obj.magic new_tls;
     new_tls
   ) else (
-    let tls = (Obj.magic tls : Obj.t array) in
+    let tls = (Obj.magic tls : view array) in
     if index < Array.length tls then
       tls
     else (
@@ -69,14 +79,17 @@ let[@inline] get_tls_ (index : int) : Obj.t array =
 let get key =
   let tls = get_tls_ key.index in
   let value = Array.unsafe_get tls key.index in
-  if value != sentinel_value_for_uninit_tls_ () then
-    Obj.magic value
-  else (
-    let value = key.compute () in
-    Array.unsafe_set tls key.index (Obj.repr (Sys.opaque_identity value));
-    value
-  )
+
+  match key.unwrap value with
+  | Some v -> v
+  | None ->
+    (match value with
+    | Sentinel ->
+      let value = key.compute () in
+      Array.unsafe_set tls key.index (key.wrap value);
+      value
+    | _ -> assert false)
 
 let set key value =
   let tls = get_tls_ key.index in
-  Array.unsafe_set tls key.index (Obj.repr (Sys.opaque_identity value))
+  Array.unsafe_set tls key.index (key.wrap value)