Vertical aligment of successive trill spanners

[Daniel Rosen]

Below an example from the NR illustrating successive trill spanners 
(http://www.lilypond.org/doc/v2.16/Documentation/af/lily-df4bb625.ly), followed 
by a slight modification. The change I've made is to add a \startTrillSpan 
command to the second whole note.

My issue is that when I compile this, the trill spanners do not all sit on the 
same baseline. How can I get them to appear more like those in the original 
example?

\version "2.16.1"

% Original:
\relative c' {
  d1\startTrillSpan
  d1
  b1\startTrillSpan
  d2\stopTrillSpan
  r2
}

% Modified:
\relative c' {
  d1\startTrillSpan
  d1\startTrillSpan
  b1\startTrillSpan
  d2\stopTrillSpan
  r2
}

DR