unterminated tie?

[Marc Weber]

.ly file:

\version "2.18.2"
<<
\context Voice = train_left_medium {
  \relative c' { g4~ b }
}
>>


|| Processing `sheet.ly'
|| Parsing...
|| Interpreting music...
sheet.ly|4 col 28| warning: unterminated tie
||           \relative c' { g4
||                            ~ b }
|| Preprocessing graphical objects...
|| Finding the ideal number of pages...
|| Fitting music on 1 page...
|| Drawing systems...
|| Layout output to `sheet.ps'...
|| Converting to `./sheet.pdf'...
|| Success: compilation successfully completed


lilypond 2.16.2 shows the same result so it must be me missunderstanding
something?

http://www.lilypond.org/doc/v2.16/Documentation/learning/ties-and-slurs
clearly says: "A tie is created by appending a tilde ~ to the first note being 
tied."

Thanks for any help

Marc Weber