[Xy-pic] Tilted ~ directional

[Michael Barr]

If you typeset the following:
\documentclass{article}
\usepackage[arrow]{xy}
  \pagestyle{empty}
\begin{document}
   $$\xy
  \POS(0,0)*+{A}\ar@{~}(0,-200)*+{B}
   \endxy$$
\end{document}
the long tilde comes out tilted.  Any explanation?  Cure?

Michael Barr