spawning a lot of processes

ring.erl

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

When I run ring:start(0) it works. But when I put a number bigger than 0 I get the following error:

Error in process <0.39.0> with exit value: {undef,[{ring,start_proc,[9,<0.31.0>]}]}

No idea what is wrong. Any help is welcome.

Forgot to export the underlying function.