Undefined values cause warnings when unions with null serialized

$enc = '';
$schema = Avro::Schema->parse(q(["long","null"]));
Avro::BinaryEncoder->encode(
    schema => $schema,
    data => undef,
    emit_cb => sub { $enc .= ${ $_[0] } },
);

Use of uninitialized value $data in pack at /home/johnkarp/git/avro/lang/perl/blib/lib/Avro/Schema.pm line 285.
Use of uninitialized value $data in string eq at /home/johnkarp/git/avro/lang/perl/blib/lib/Avro/Schema.pm line 287.