diff --git a/lib/picos_std.structured/bundle.ml b/lib/picos_std.structured/bundle.ml
index b1b3593c..0fed3e64 100644
--- a/lib/picos_std.structured/bundle.ml
+++ b/lib/picos_std.structured/bundle.ml
@@ -5,13 +5,17 @@ let[@inline never] completed () = invalid_arg "already completed"
 type _ tdt =
   | Nothing : [> `Nothing ] tdt
   | Bundle : {
-      config : int Atomic.t;
+      mutable _config : int;
       bundle : Computation.packed;
       errors : Control.Errors.t;
       finished : Trigger.t;
     }
       -> [> `Bundle ] tdt
 
+type t = [ `Bundle ] tdt
+
+external config_as_atomic : t -> int Atomic.t = "%identity"
+
 let config_terminated_bit = 0x01
 and config_callstack_mask = 0x3E
 and config_callstack_shift = 1
@@ -19,8 +23,6 @@ and config_one = 0x40 (* memory runs out before overflow *)
 
 let flock_key : [ `Bundle | `Nothing ] tdt Fiber.FLS.t = Fiber.FLS.create ()
 
-type t = [ `Bundle ] tdt
-
 let terminate ?callstack (Bundle { bundle = Packed bundle; _ } : t) =
   Computation.cancel bundle Control.Terminate
     (Control.get_callstack_opt callstack)
@@ -36,8 +38,8 @@ let error ?callstack (Bundle r as t : t) exn bt =
     Control.Errors.push r.errors exn bt
   end
 
-let decr (Bundle r : t) =
-  let n = Atomic.fetch_and_add r.config (-config_one) in
+let decr (Bundle r as t : t) =
+  let n = Atomic.fetch_and_add (config_as_atomic t) (-config_one) in
   if n < config_one * 2 then begin
     let (Packed bundle) = r.bundle in
     Computation.cancel bundle Control.Terminate Control.empty_bt;
@@ -72,9 +74,8 @@ let[@inline never] raised exn t fiber packed canceler outer =
   await t fiber packed canceler outer;
   Printexc.raise_with_backtrace exn bt
 
-let[@inline never] returned value (Bundle r as t : t) fiber packed canceler
-    outer =
-  let config = Atomic.get r.config in
+let[@inline never] returned value (t : t) fiber packed canceler outer =
+  let config = Atomic.get (config_as_atomic t) in
   if config land config_terminated_bit <> 0 then begin
     let callstack =
       let n = (config land config_callstack_mask) lsr config_callstack_shift in
@@ -108,11 +109,11 @@ let join_after_pass (type a) ?callstack ?on_return (fn : a -> _) (pass : a pass)
             Int.min n (config_callstack_mask lsr config_callstack_shift)
             lsl config_callstack_shift
     in
-    let config = Atomic.make (config_one lor callstack lor terminated) in
+    let config = config_one lor callstack lor terminated in
     let bundle = Computation.Packed (Computation.create ~mode:`LIFO ()) in
     let errors = Control.Errors.create () in
     let finished = Trigger.create () in
-    Bundle { config; bundle; errors; finished }
+    Bundle { _config = config; bundle; errors; finished }
   in
   let fiber = Fiber.current () in
   let outer = Fiber.FLS.get fiber flock_key ~default:Nothing in
@@ -130,10 +131,12 @@ let join_after_pass (type a) ?callstack ?on_return (fn : a -> _) (pass : a pass)
   in
   join_after_realloc x fn t fiber packed canceler outer
 
-let rec incr (Bundle r as t : t) backoff =
-  let before = Atomic.get r.config in
+let rec incr (t : t) backoff =
+  let before = Atomic.get (config_as_atomic t) in
   if before < config_one then completed ()
-  else if not (Atomic.compare_and_set r.config before (before + config_one))
+  else if
+    not
+      (Atomic.compare_and_set (config_as_atomic t) before (before + config_one))
   then incr t (Backoff.once backoff)
 
 let finish (Bundle { bundle = Packed bundle; _ } as t : t) canceler =
@@ -214,5 +217,5 @@ let join_after ?callstack ?on_return fn =
 let fork t thunk = fork_pass t thunk Arg
 let fork_as_promise t thunk = fork_as_promise_pass t thunk Arg
 
-let unsafe_incr (Bundle r : t) =
-  Atomic.fetch_and_add r.config config_one |> ignore
+let unsafe_incr (t : t) =
+  Atomic.fetch_and_add (config_as_atomic t) config_one |> ignore