Ties in LilyPond 1.1.25

[Peter Chubb]

Hi,
        How do I produce a tie to one note of a chord?

I've tried both obvious ways:

\notes \relative c{\clef "bass"; a2 < ~a a,> | <{a2 ~ a2} { s2 a,2}>}
but both give me: 
        warning: Unequal number of note heads for tie:
and instead of a tied `a' I get a slur from a to a, in each case.

Even
        a2( < )a2 a,2>

gives the wrong result.

PeterC