In divisi lyrics, \skip in the top line skips two notes and causes bottom line to skip one note

[Edward Usher]

A question on the following MWE:

\version "2.18.2"
\score {
  <<
    \new Voice = "melody" {
      \time 3/4
      c'4 c'4 c'4 | c'4 c'4 c'4 | c'2. |
    }
    \new Lyrics \lyricsto "melody" {
      <<
        { a \skip 1 c }
        \new Lyrics {
          \set associatedVoice = "melody"
          a b c d
        }
      >>
    }
  >>
}

I expected the notes to be labeled "a [gap] c [gap] [gap] [gap] [gap]" on the first line and "a b c d [gap] [gap] [gap]" on the second.  But the actual output is "a [gap] [gap] c [gap] [gap] [gap]" and "a b c [gap] d [gap] [gap]".

Is there an error in the MWE, or is this a bug?  If a bug, is there a workaround?  (I can't use "  " in place of \skip 1 in the actual score because I need to terminate a melisma.)

Edward