Coercing a type that can be undef

[Octavian Rasnita]

Hello,

I am trying to create a module that accepts a string property as 2010-05-01 and 
coerces it to DateTime by default, but that property might be also undef.

I have tried:

package Foo;

use Moose;
use Moose::Util::TypeConstraints;
use DateTime;
use DateTime::Format::Natural;

subtype 'BRK::DT' => as class_type('DateTime');

coerce 'BRK::DT' => from 'Str' => via 
{DateTime::Format::Natural->new->parse_datetime($_)};

has start => (is => 'rw', isa => 'Maybe[BRK::DT]', coerce => 1);

package main;

my $foo = Foo->new;

my $date;
$date = '2010-05-01';

$foo->start($date);

print $foo->start->month_name;

__END__

This program works, but if I comment the following line:
$date = '2010-05-01';

than it gives the error:

Cannot coerce without a type coercion at 
e:/usr/site/lib/Moose/Meta/TypeConstraint.pm line 83
 
Moose::Meta::TypeConstraint::coerce('Moose::Meta::TypeConstraint::Parameterized=HASH(0x2570c8c)',
 '2010-05-01') called at accessor start defined at E:\lucru\moose\test8.pl line 
16
 Foo::start('Foo=HASH(0x256de04)', '2010-05-01') called at 
E:\lucru\moose\test8.pl line 20

Thank you for your help.

Octavian