diff --git a/lib/polly.ml b/lib/polly.ml
index b96c5db..146ec4e 100644
--- a/lib/polly.ml
+++ b/lib/polly.ml
@@ -40,14 +40,19 @@ end
 
 type t = Unix.file_descr (* epoll fd *)
 
-external caml_polly_add : t -> Unix.file_descr -> Events.t -> unit
+external caml_raise_unix_error : string -> 'a = "caml_raise_unix_error"
+
+external caml_polly_add : t -> Unix.file_descr -> Events.t -> int
   = "caml_polly_add"
+[@@noalloc]
 
-external caml_polly_del : t -> Unix.file_descr -> Events.t -> unit
+external caml_polly_del : t -> Unix.file_descr -> Events.t -> int
   = "caml_polly_del"
+[@@noalloc]
 
-external caml_polly_mod : t -> Unix.file_descr -> Events.t -> unit
+external caml_polly_mod : t -> Unix.file_descr -> Events.t -> int
   = "caml_polly_mod"
+[@@noalloc]
 
 external caml_polly_create1 : unit -> t = "caml_polly_create1"
 
@@ -70,11 +75,17 @@ let create = caml_polly_create1
 
 let close t = Unix.close t
 
-let add = caml_polly_add
+let add t fd e =
+  let r = caml_polly_add t fd e in
+  if r < 0 then caml_raise_unix_error "Polly.add"
 
-let del t fd = caml_polly_del t fd Events.empty
+let del t fd =
+  let r = caml_polly_del t fd Events.empty in
+  if r < 0 then caml_raise_unix_error "Polly.del"
 
-let upd = caml_polly_mod
+let upd t fd e =
+  let r = caml_polly_mod t fd e in
+  if r < 0 then caml_raise_unix_error "Polly.mod"
 
 let wait = caml_polly_wait
 
diff --git a/lib/polly_stubs.c b/lib/polly_stubs.c
index cb590bb..c953c19 100644
--- a/lib/polly_stubs.c
+++ b/lib/polly_stubs.c
@@ -35,19 +35,25 @@ CAMLprim value caml_polly_create1(value val_unit)
 	CAMLreturn(val_res);
 }
 
+/* beware, uerror is a macro in 5.0 and a function in 4.X, the new function
+   name is caml_uerror. uerror might be deprecated in the future. */
+CAMLprim void caml_raise_unix_error(value funname) {
+  CAMLparam1(funname);
+  uerror(String_val(funname),Nothing);
+}
+
+
 static value
 caml_polly_ctl(value val_epfd, value val_fd, value val_events, int op)
 {
-	CAMLparam3(val_epfd, val_fd, val_events);
-	struct epoll_event event = {
-		.events = (uint32_t) Int_val(val_events),
-		.data.fd = Int_val(val_fd)
-	};
+  CAMLparam3(val_epfd, val_fd, val_events);
+  struct epoll_event event = {
+    .events = (uint32_t) Int_val(val_events),
+    .data.fd = Int_val(val_fd)
+  };
 
-	if (epoll_ctl(Int_val(val_epfd), op, Int_val(val_fd), &event) == -1)
-		uerror(__FUNCTION__, Nothing);
 
-	CAMLreturn(Val_unit);
+  CAMLreturn(Val_int(epoll_ctl(Int_val(val_epfd), op, Int_val(val_fd), &event)));
 }
 
 CAMLprim value caml_polly_add(value val_epfd, value val_fd, value val_events)