[XeTeX] Using tipa.sty with XeLaTeX

[R (Chandra) Chandrasekhar]

Dear Folks,

I am trying to compile a file with IPA phonetic characters originally typeset 
with the tipa.sty package and compiled with LaTeX.  I am now trying to compile 
it with XeLaTeX but have run into problems.

To illustrate the problem, a minimal example file is given below:
----------
\documentclass[a4paper,10pt]{article}
%
\usepackage{fontspec}
\usepackage{xunicode}
\usepackage{xltxtra}
\usepackage{tipa}
%
\defaultfontfeatures{Mapping=tex-text,Scale=MatchLowercase,}
\setromanfont[Numbers=OldStyle]{Minion Pro}
\setsansfont[Numbers=OldStyle]{Myriad Pro}
\setmonofont{DejaVu Sans Mono}
%
\begin{document}

\noindent \textbf{thesis}/\textipa{"Ti:sIs}/ and
\noindent\textbf{hypothesis}/\textipa{h2I"p6TIsIs}/

\end{document}
----------

Upon being compiled with xelatex, it chokes with the message:
----------
! Undefined control sequence.
2->\textezh

l.16 ...t\textbf{hypothesis}/\textipa{h2I"p6TIsIs}
                                               /
----------

FYI, I am using TeXLive 2007 on a Kubuntu installation with fontspec version 
[2006/12/24 v1.13 Advanced font selection for XeLaTeX].

How should I proceed please?

Thanks.

Chandra
10 Sep 07