diff --git a/sqlexpr_sqlite_lwt.ml b/sqlexpr_sqlite_lwt.ml
index e3061eb..736f37d 100644
--- a/sqlexpr_sqlite_lwt.ml
+++ b/sqlexpr_sqlite_lwt.ml
@@ -41,12 +41,13 @@ struct
     mutable thread : Thread.t;
     mutable finished : bool;
     db : db; (* keep ref to db so it isn't GCed while a worker is active *)
+    mutex : Lwt_mutex.t;
   }
 
   type stmt = worker * Stmt.t
   type 'a result = 'a Lwt.t
 
-  let max_threads = ref 100
+  let max_threads = ref 4
   let set_default_max_threads n = max_threads := n
 
   let close_db db =
@@ -120,6 +121,7 @@ struct
         task_channel = Event.new_channel ();
         thread = Thread.self ();
         finished = false;
+        mutex = Lwt_mutex.create ();
       }
     in worker.thread <- Thread.create (worker_loop worker) ();
        worker
@@ -169,11 +171,12 @@ struct
                  wakeup wakener value
              | `Failure exn ->
                  wakeup_exn wakener exn)
-    in try_lwt
-      check_worker_finished worker;
-      (* Send the id and the task to the worker: *)
-      Event.sync (Event.send worker.task_channel (id, task));
-      waiter
+    in Lwt_mutex.with_lock worker.mutex
+         (fun () -> try_lwt
+            check_worker_finished worker;
+            (* Send the id and the task to the worker: *)
+            Event.sync (Event.send worker.task_channel (id, task));
+            waiter)
 
   let do_raise_error ?sql ?params ?errmsg errcode =
     let msg = Sqlite3.Rc.to_string errcode ^ Option.map_default ((^) " ") "" errmsg in
@@ -247,15 +250,15 @@ struct
              List.iter
                (fun v -> match Stmt.bind stmt !n v with
                     Sqlite3.Rc.OK -> decr n
-                  | code -> do_raise_error ~sql ~params code)
+                  | code -> add_worker db worker; do_raise_error ~sql ~params code)
                params)
         stmt >>
       profile_execute_sql sql ~params
         (fun () ->
            try_lwt
+             add_worker db worker;
              f (worker, stmt) sql params
            finally
-             add_worker db worker;
              match stmt_id with
                  Some id -> Stmt_cache.add_stmt worker.stmt_cache id stmt; return ()
                | None -> return ())