[XeTeX] Problem using XeTeXinterchartoks

[Barry MacKichan]

I am having trouble using XeTeXinterchartoks. My TeX file is below, and 
the resulting PDF file is attached. The Chinese characters appear in the 
first line (showing that \zhfam is defined correctly), but in the second 
line, there is blank space where the Chinese characters should be. Any 
help would be appreciated, such as a correction of this file or a link 
to a complete working sample.

I am using XeTeX version 0.997 svn 539, the one included in the MikTeX 
download.

Thanks,

Barry MacKichan

\documentclass{article}%
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{fontspec}
\usepackage{xltxtra}
\usepackage{xunicode}

\newfontfamily\zhfam{MingLiU}
\XeTeXinterchartokenstate = 1
\XeTeXinterchartoks 0 1 {\begingroup\zhfam}
\XeTeXinterchartoks 1 0 {\endgroup}
\XeTeXinterchartoks 255 1 {\begingroup\zhfam}
\XeTeXinterchartoks 1 255 {\endgroup}

\begin{document}
Here is some text: \begingroup\zhfam 多項式函數: \endgroup
\\
Now is the time to write: 多項式函數:
\\
\end{document}
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