Failing Erlang monitor

Console output

Erlang R15B (erts-5.9) [source] [64-bit] [smp:4:4] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.9  (abort with ^G)
1> c(doubler).
{ok,doubler}
2> c(monitor).
{ok,monitor}
3> 
3> monitor:start().
new
4> doubler:double(doubler, 2).
4
5> doubler:double(doubler, "").
<0.44.0> exited: {badarith,[{doubler,loop,0,[{file,"doubler.erl"},{line,7}]}]}.
=ERROR REPORT==== 29-Apr-2012::20:22:52 ===
Error in process <0.44.0> with exit value: {badarith,[{doubler,loop,0,[{file,"doubler.erl"},{line,7}]}]}

...and there it stops, and I have to kill the console. The monitor seems to be restarting it OK, but I'm obviously doing something wrong.

doubler.erl

-module(doubler).
-export([loop/0, double/2]).

loop() ->
  receive
    {Pid, N} ->
      Pid ! (N * 2),
      loop()
end.

double(To, N) ->
  To ! {self(), N},
  receive
    Result -> Result
  end.

monitor.erl

-module(monitor).
-export([loop/0, start/0]).

loop() ->
  process_flag(trap_exit, true),
  receive
    new ->
      register(doubler, spawn_link(fun doubler:loop/0)),
      loop();
    {'EXIT', From, Reason} ->
      io:format("~p exited: ~p.", [From, Reason]),
      monitor ! new,
      loop()
end.

start() ->
  register(monitor, spawn(fun monitor:loop/0)),
  monitor ! new.