alternative repeat - extraneous barline

[Ken Pierce]

An extraneous barline appears in the middle of the second ending.  How can I
eliminate it?

Thanks,
Ken

--- example:

\version "2.18.2"
\score {
   \new Staff {
       \time 6/8 \partial 8
       \repeat volta 2 { a'8 | a'8. g'16 f'8 a'8. g'16 f'8 | }
       \alternative {
         {  d'4.~ d'4 }
         {  d'4.~ d'4 } % extraneous barline appears
       }
      }
   \layout { }
}