Unexpected "warning: unterminated tie"

[Ole V . Villumsen]

> I'm not top posting.

This may not be strictly by the official recommendations. However, I had
expected it to work.

\version "2.18.2"

% expected: ties g'-g', b-b and g-g
% actual: "warning: unterminated tie", no tie g'-g', ties b-b and g-g

\relative c' {
  << { g'2 ~ g } { s4 b, ~ b2 } \\ { r8 d b g ~ g2 } >>
}

At the first tilde (~) in the input, Lilypond 2.18.2 says "warning:
unterminated tie", and indeed that tie is missing from the output.

(The music is simplified from Bourree II of J.S. Bach: French Overture BWV 831a)

I got even more surprised (and still unpleasantly so) when removing the
lower voice and trying with just << { g'2 ~ g } { s4 b, ~ b2 } >>. Now the
music got typeset on two staves. Both the expected ties were there, though,
one on each staff.