Attribute is => 'Maybe[SomeSubtype]' returns Attribute () does not pass type constraint

I've created subtype Birth_d with coercion as shown below, and I'm trying to use it in combination with the built-in Maybe type, per Moose::Manual::Types.

I'm getting the error You cannot coerce an attribute (birth_d) unless its type (Maybe[Birth_d]) has a coercion. Here's complete test code:

package Student;
use Moose;
use Moose::Util::TypeConstraints;

use DateTime::Format::MySQL;

class_type 'Birth_d', { class => 'DateTime' };

coerce 'Birth_d',
  from 'Str',
  via { DateTime::Format::MySQL->parse_date( $_ ) };

has 'name' => (
  isa => 'Str',
  is => 'ro',
);

has 'birth_d' => (
  isa => 'Maybe[Birth_d]',   # This works:    isa => 'Birth_d'
  coerce => 1,
  is => 'ro',
);


package main;

use Test::More;

my $student = Student->new(
  name => 'Johnnie Appleseed',
  birth_d => '2015-01-01'
  );

is ( $student->birth_d->ymd(), '2015-01-01' );

my $student2 = Student->new(
  name => 'Foo Bar',
  birth_d => undef
  );

is( $student2->birth_d, undef );

Replacing isa => 'Maybe[Birth_d]' with isa => 'Birth_d' works, but is not what is needed. I need to make the birth_d optional, and if not supplied, should be undef.

I should add, I tried using MooseX::Types to tuck this Birth_d type away in a separate place, but found its cavalier use of barewords a bit unorthodox, so I slowly backed away. I'm open to reconsidering it, if it makes sense to do so.