diff --git a/ocaml/message-switch/async/protocol_async.ml b/ocaml/message-switch/async/protocol_async.ml
index 3575ac268bf..636cfd5e2b4 100644
--- a/ocaml/message-switch/async/protocol_async.ml
+++ b/ocaml/message-switch/async/protocol_async.ml
@@ -82,13 +82,13 @@ module M = struct
 
     let with_lock t f =
       let rec wait state =
-        if Bool.(t.m = state) then
+        if Bool.(t.m = state) then (
+          t.m <- true ;
           return ()
-        else
+        ) else
           Condition.wait t.c >>= fun () -> wait state
       in
       wait false >>= fun () ->
-      t.m <- true ;
       Monitor.protect f ~finally:(fun () ->
           t.m <- false ;
           Condition.broadcast t.c () ;