MX::Method with Maybe[] not working?

I'm not getting what I expect when I use Maybe[] with MX::Method. Am I
misunderstanding this, or doing something wrong?

I have a method that needs to accept either a scalar value or undefined.
With 'Value', it accepts any defined scalar. But with 'Maybe[Value]', it
rejects all scalars.

Method is defined as:

  method 'validate' => positional( { isa => 'Maybe[Value]', required =>
1 } ) => sub { 1; };

No matter whether I pass in a scalar or undefined value, I get:

  Parameter 0: Argument isn't (Maybe[Value])

Poking in the debugger, it seems to show that 'Maybe[Value]' requires an
Object:

  DB<2> x $self->{'@!parameter_map'}
0  ARRAY(0x130b0d0)
   0  MooseX::Meta::Parameter=HASH(0x1305240)
      'isa' => 'Maybe[Value]'
      'required' => 1
      'type_constraint' => Moose::Meta::TypeConstraint=HASH(0x13137b0)
         'compiled_type_constraint' => CODE(0x1313770)
            -> &CODE(0x1313770) in ???
         'constraint' => CODE(0x1305280)
            ->
&MooseX::Meta::Parameter::__ANON__[/usr/share/perl5/MooseX/Meta/Parameter.pm:34]
in /usr/share/perl5/MooseX/Meta/Parameter.pm:34-34
         'name' => '__ANON__'
         'package_defined_in' => 'MooseX::Meta::Parameter'
         'parent' => Moose::Meta::TypeConstraint=HASH(0x10c3c20)
            'compiled_type_constraint' => CODE(0x10a3250)
               ->
&Moose::Util::TypeConstraints::OptimizedConstraints::Object in
/usr/share/perl5/Moose/Util/TypeConstraints/OptimizedConstraints.pm:33-33
            'constraint' => CODE(0x10eddc0)
               ->
&Moose::Util::TypeConstraints::__ANON__[/usr/share/perl5/Moose/Util/TypeConstraints.pm:453]
in /usr/share/perl5/Moose/Util/TypeConstraints.pm:453-453
            'hand_optimized_type_constraint' => CODE(0x10a3250)
               -> REUSED_ADDRESS
            'name' => 'Object'
            'package_defined_in' => 'Moose::Util::TypeConstraints'
            'parent' => Moose::Meta::TypeConstraint=HASH(0x10c3540)
               'compiled_type_constraint' => CODE(0x1091ee0)
                  ->
&Moose::Util::TypeConstraints::OptimizedConstraints::Ref in
/usr/share/perl5/Moose/Util/TypeConstraints/OptimizedConstraints.pm:13-13
               'constraint' => CODE(0x10df150)
                  ->
&Moose::Util::TypeConstraints::__ANON__[/usr/share/perl5/Moose/Util/TypeConstraints.pm:418]
in /usr/share/perl5/Moose/Util/TypeConstraints.pm:418-418
               'hand_optimized_type_constraint' => CODE(0x1091ee0)
                  -> REUSED_ADDRESS
               'name' => 'Ref'
               'package_defined_in' => 'Moose::Util::TypeConstraints'
               'parent' => Moose::Meta::TypeConstraint=HASH(0x10c39a0)
                  'compiled_type_constraint' => CODE(0x10c3570)
                     -> &Moose::Meta::TypeConstraint::Defined in 0
                  'constraint' => CODE(0x10ded00)
                     ->
&Moose::Util::TypeConstraints::__ANON__[/usr/share/perl5/Moose/Util/TypeConstraints.pm:405]
in /usr/share/perl5/Moose/Util/TypeConstraints.pm:405-405
                  'name' => 'Defined'
                  'package_defined_in' => 'Moose::Util::TypeConstraints'
                  'parent' =>
Moose::Meta::TypeConstraint=HASH(0x10c35c0)
                     'compiled_type_constraint' => CODE(0x10c3910)
                        -> &Moose::Meta::TypeConstraint::Item in 0
                     'constraint' => CODE(0x10deac0)
                        ->
&Moose::Util::TypeConstraints::__ANON__[/usr/share/perl5/Moose/Util/TypeConstraints.pm:402]
in /usr/share/perl5/Moose/Util/TypeConstraints.pm:402-402
                     'name' => 'Item'
                     'package_defined_in' =>
'Moose::Util::TypeConstraints'
  DB<3> T
. = eval {...} called from file
`/usr/share/perl5/MooseX/Meta/Signature/Positional.pm' line 43
@ =
MooseX::Meta::Signature::Positional::validate(ref(MooseX::Meta::Signature::Positional),
0) called from file `/usr/share/perl5/MooseX/Method.pm' line 85
. = eval {...} called from file `/usr/share/perl5/MooseX/Method.pm' line
84
. = DataSmithy::Role::DataType::validate(ref(Testme), 0) called from
file `t/030-datasmithy-role-datatype.t' line 46
. = main::__ANON__[t/030-datasmithy-role-datatype.t:46]() called from
file `/usr/share/perl5/Sub/Uplevel.pm' line 102
. = Sub::Uplevel::uplevel(3, ref(CODE)) called from file
`/usr/share/perl5/Test/Exception.pm' line 89
. = eval {...} called from file `/usr/share/perl5/Test/Exception.pm'
line 89
$ = Test::Exception::_try_as_caller(ref(CODE)) called from file
`/usr/share/perl5/Test/Exception.pm' line 243
. = Test::Exception::lives_ok(ref(CODE), '->validate succeeds with
undef') called from file `t/030-datasmithy-role-datatype.t' line 46


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