Problem with ring in erlang :/

ring.erl

-module(ring).
-export([start/1,start_proc/2]).

start(Num) ->
    start_proc(Num, self()).

start_proc(0,Pid) ->
    Pid ! ok;
start_proc(Num, Pid) ->
    NPid = spawn(ring, start_proc, [Num -1, Pid]),
    NPid ! ok,
    receive 
        ok -> ok
    end.

shell

Eshell V5.8.3  (abort with ^G)
1> c(ring).
{ok,ring}
2> timer:tc(ring,start,[1000]).

=ERROR REPORT==== 10-Feb-2012::15:45:35 ===
Error in process <0.38.0> with exit value: {undef,[{ring,start_proc,[999,<0.31.0>]}]}