[XeTeX] Undefined control sequence

[Cesar Romani]

I'm using xelatex from TexLive 2008 on Win XP.
I wanted to run the following text under xelatex:

--------------------
\documentclass{article}

\usepackage{fontspec}
\usepackage{xunicode}
\usepackage{xltxtra}
\usepackage{polyglossia}

\newfontfamily\devanagarifont[Script=Devanagari]{Nakula}
\newfontfamily\englishfont[Script=Latin]{XITS}

\setmainlanguage{sanskrit}
\setotherlanguages{english}

\begin{document}

\section{संस्कृतम्}
जीवनस्य लक्ष्यमेव संस्कृतस्य वर्धनम्
\subsection{कोऽम्}
\textenglish{(Sanskrit)}
\end{document}
--------------------


Bu I'm getting the following error:

--------------------
! Undefined control sequence.
\zf at check@ot at lang ...Tcountlanguages \zf at basefont
                                                   \c at zf@script 
\c at zf@index \...
l.14 \begin{document}
--------------------

Many thanks in advance,

-- 
Cesar