modalTranspose with chromatic

[address@hidden]

I'm having a problem with chromatic scales and \modalTranspose.  According to: 
http://lilypond.org/doc/v2.18/Documentation/notation/changing-multiple-pitches#modal-transformations
 it should be possible to do what I want.  Ascending passages work as expected. 
 Descending ones do not.  I get an unexpected octave displacement.  The 
following code produces the odd result.  What I want is for each descending 
motive to simply descend a chromatic octave without leaping up an octave.  What 
is going on and how can I achieve my desired result?

-David

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\version "2.18.0"

chromaticScaleUp = \relative c { c cis d dis e f fis g gis a ais b }

chromaticScaleDown = \relative c { c b bes a as g ges f e es d des }

motiveUp = \relative c {
  c8 cis d dis e f
  fis g gis a ais b
  c4. r
}

motiveDown = \relative c' {
  c8 b bes a as g
  ges f e es d des
  c4. r
}

\score {
  \relative {
    \clef F
  \time 6/8
  \modalTranspose c g \chromaticScaleUp \motiveUp
  \modalTranspose c g \chromaticScaleDown \motiveDown
  }
}

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