[Dbix-class] can't locate object method "union" when using DBIx::Class::Helper::ResultSet::SetOperations

[x li]

http://search.cpan.org/~frew/DBIx-Class-Helpers-2.007000/lib/DBIx/Class/Helper/ResultSet/SetOperations.pm

my $rs1 = $schema->resultset('ContrastDataSetAnnotation')->search(undef, {
    columns => [qw( name value )],
});
my $rs2 = $schema->resultset('GeneSetAnnotation')->search(undef, {
    columns => [qw( name value )],
});

$_->result_class('MyApp::Schema::Result::ContrastDataSetAnnotation')
#  add this
          for ($rs1, $rs2);

my @annotations = $rs1->union($rs2)->all;