Differently headed does not merge.

[Peter Terpstra]

Dear People,
Found a problem with merging differently headed in the next example:

\version "2.18.2"
\relative c {
  \key d \major
  \clef "treble_8"
  \mergeDifferentlyHeadedOn
  \mergeDifferentlyDottedOn
<< { <a''-2>2 ~ a8<d-4>8<cis-3><a-4> }\\
   { d,,,8 a' <fis'-2><cis'-3> b2 }\\
   { d,,2 s } >>
}

Is there a solution to this?

Many thanks in advantage.

Peter