diff --git a/lib/task.ml b/lib/task.ml
index 3599925..30b9caf 100644
--- a/lib/task.ml
+++ b/lib/task.ml
@@ -4,7 +4,9 @@ open Effect.Deep
 type 'a task = unit -> 'a
 
 type message =
-  Work of (unit -> unit)
+| Work of (unit -> unit)
+  (* Invariant: the Work function does not need to run under the 'step' handler,
+     it installs its own handler or re-invokes a deep-handler continuation. *)
 | Quit
 
 type task_chan = message Multi_channel.t
@@ -48,12 +50,6 @@ let do_task (type a) (f : unit -> a) (p : a promise) : unit =
   | Pending l -> List.iter action l
   |  _ -> failwith "Task.do_task: impossible, can only set result of task once"
 
-let async pool f =
-  let pd = get_pool_data pool in
-  let p = Atomic.make (Pending []) in
-  Multi_channel.send pd.task_chan (Work (fun _ -> do_task f p));
-  p
-
 let await pool promise =
   let pd = get_pool_data pool in
   match Atomic.get promise with
@@ -78,10 +74,16 @@ let step (type a) (f : a -> unit) (v : a) : unit =
           loop ())
       | _ -> None }
 
+let async pool f =
+  let pd = get_pool_data pool in
+  let p = Atomic.make (Pending []) in
+  Multi_channel.send pd.task_chan (Work (fun _ -> step (do_task f) p));
+  p
+
 let rec worker task_chan =
   match Multi_channel.recv task_chan with
   | Quit -> Multi_channel.clear_local_state task_chan
-  | Work f -> step f (); worker task_chan
+  | Work f -> f (); worker task_chan
 
 let run (type a) pool (f : unit -> a) : a =
   let pd = get_pool_data pool in
@@ -93,7 +95,7 @@ let run (type a) pool (f : unit -> a) : a =
         begin
           try
             match Multi_channel.recv_poll pd.task_chan with
-            | Work f -> step f ()
+            | Work f -> f ()
             | Quit -> failwith "Task.run: tasks are active on pool"
           with Exit -> Domain.cpu_relax ()
         end;