diff --git a/log.ml b/log.ml
index b9285ab..e704da6 100644
--- a/log.ml
+++ b/log.ml
@@ -18,13 +18,15 @@
 open Lwt
 open Sexp
 
+let mtx = Lwt_mutex.create ()
 let write_to_log label body =
-  try_lwt
-    lwt () = Lwt_io.print label in
-    lwt () = Lwt_io.print ": " in
-    lwt () = output_sexp_human Lwt_io.stdout body in
-    Lwt_io.printl ""
-  with _ -> return ()
+  Lwt_mutex.with_lock mtx (fun () ->
+    try_lwt
+      lwt () = Lwt_io.print label in
+      lwt () = Lwt_io.print ": " in
+      lwt () = output_sexp_human Lwt_io.stdout body in
+      Lwt_io.printl ""
+    with _ -> return ())
 
 let hook = ref write_to_log