How to get the hairpin to the barline?

[Pieter Terpstra]

Dear people,
Have looked this up in the lilypond doc and it work in the example there but no 
in mine, which is a bit different.

How do i do this?

Example:
\version "2.18.2"

\score {
\relative c'' {
  \mergeDifferentlyHeadedOn \mergeDifferentlyDottedOn
   \override Hairpin.to-barline = ##t
<<  { 
  \voiceOne
   a,8 <a' cis><a cis><g b>_(<a cis>)^\><a cis>4.\! |
   } \\ {
   \voiceTwo
   a,2. a8 (b) | 
   } >>  
}}

Thank you so much in advance!

Peter