diff --git a/src/ws_pool.ml b/src/ws_pool.ml
index 874cbd5c..d32c71f8 100644
--- a/src/ws_pool.ml
+++ b/src/ws_pool.ml
@@ -5,7 +5,16 @@ include Runner
 
 let ( let@ ) = ( @@ )
 
+module Id = struct
+  type t = unit ref
+  (** Unique identifier for a pool *)
+
+  let create () : t = Sys.opaque_identity (ref ())
+  let equal : t -> t -> bool = ( == )
+end
+
 type worker_state = {
+  pool_id_: Id.t;  (** Unique per pool *)
   mutable thread: Thread.t;
   q: task WSQ.t;  (** Work stealing queue *)
   rng: Random.State.t;
@@ -17,6 +26,7 @@ type worker_state = {
 type around_task = AT_pair : (t -> 'a) * (t -> 'a -> unit) -> around_task
 
 type state = {
+  id_: Id.t;
   active: bool A.t;  (** Becomes [false] when the pool is shutdown. *)
   workers: worker_state array;  (** Fixed set of workers. *)
   main_q: task Queue.t;  (** Main queue for tasks coming from the outside *)
@@ -59,7 +69,10 @@ let schedule_task_ (self : state) (w : worker_state option) (task : task) : unit
     =
   (* Printf.printf "schedule task now (%d)\n%!" (Thread.id @@ Thread.self ()); *)
   match w with
-  | Some w ->
+  | Some w when Id.equal self.id_ w.pool_id_ ->
+    (* we're on this same pool, schedule in the worker's state. Otherwise
+       we might also be on pool A but asking to schedule on pool B,
+       so we have to check that identifiers match. *)
     let pushed = WSQ.push w.q task in
     if pushed then
       try_wake_someone_ self
@@ -70,7 +83,7 @@ let schedule_task_ (self : state) (w : worker_state option) (task : task) : unit
       if self.n_waiting_nonzero then Condition.signal self.cond;
       Mutex.unlock self.mutex
     )
-  | None ->
+  | _ ->
     if A.get self.active then (
       (* push into the main queue *)
       Mutex.lock self.mutex;
@@ -216,6 +229,7 @@ let dummy_task_ () = assert false
 let create ?(on_init_thread = default_thread_init_exit_)
     ?(on_exit_thread = default_thread_init_exit_) ?(on_exn = fun _ _ -> ())
     ?around_task ?num_threads () : t =
+  let pool_id_ = Id.create () in
   (* wrapper *)
   let around_task =
     match around_task with
@@ -233,6 +247,7 @@ let create ?(on_init_thread = default_thread_init_exit_)
     let dummy = Thread.self () in
     Array.init num_threads (fun i ->
         {
+          pool_id_;
           thread = dummy;
           q = WSQ.create ~dummy:dummy_task_ ();
           rng = Random.State.make [| i |];
@@ -241,6 +256,7 @@ let create ?(on_init_thread = default_thread_init_exit_)
 
   let pool =
     {
+      id_ = pool_id_;
       active = A.make true;
       workers;
       main_q = Queue.create ();
diff --git a/test/dune b/test/dune
index 56261dad..43955ec6 100644
--- a/test/dune
+++ b/test/dune
@@ -1,6 +1,7 @@
 (tests
  (names
   t_fib
+  t_ws_pool_confusion
   t_bench1
   t_fib_rec
   t_futs1
diff --git a/test/t_ws_pool_confusion.ml b/test/t_ws_pool_confusion.ml
new file mode 100644
index 00000000..20488b65
--- /dev/null
+++ b/test/t_ws_pool_confusion.ml
@@ -0,0 +1,28 @@
+open Moonpool
+
+let delay () = Thread.delay 0.001
+
+let run ~p_main:_ ~p_sub () =
+  let f1 =
+    Fut.spawn ~on:p_sub (fun () ->
+        delay ();
+        1)
+  in
+  let f2 =
+    Fut.spawn ~on:p_sub (fun () ->
+        delay ();
+        2)
+  in
+  Fut.wait_block_exn f1 + Fut.wait_block_exn f2
+
+let () =
+  let p_main = Ws_pool.create ~num_threads:2 () in
+  let p_sub = Ws_pool.create ~num_threads:10 () in
+
+  let futs = List.init 8 (fun _ -> Fut.spawn ~on:p_main (run ~p_main ~p_sub)) in
+
+  let l = List.map Fut.wait_block_exn futs in
+  assert (l = List.init 8 (fun _ -> 3));
+
+  print_endline "ok";
+  ()