another puppet bug?

puppet apply test1.pp .sh

$ puppet apply test.pp                                                                                                      
notice: result:one-two
notice: /Stage[main]/Foo/Notify[result:one-two]/message: defined 'message' as 'result:one-two'
notice: Finished catalog run in 0.03 seconds

puppet apply test2.pp .sh

notice: result:-two
notice: /Stage[main]/Foo/Notify[result:-two]/message: defined 'message' as 'result:-two'
notice: Finished catalog run in 0.03 seconds

test1.pp

class foo (
    $x          = 'one',
    $y        = "${x}-two"
  ) {

  notify { "result:${y}" : }
}

include foo

test2.pp

class foo (
    $xx          = 'one',
    $yy        = "${xx}-two"
  ) {

  notify { "result:${yy}" : }
}

include foo