Problem with xelatex [closed]

Question

I get the error

! Argument of \tl_to_str_aux:w has an extra }.
<inserted text> 
                \par 
l.61 \tl_set:Nx \c_colon_str { \tl_to_str:N : }

when I try to format

\documentclass{article}
\usepackage{fontspec}
\setmainfont{Arial}
\begin{document}
The quick brown fox jumps over the lazy dog.
\textbf{The quick brown fox jumps over the lazy dog.}
\emph{The quick brown fox jumps over the lazy dog.}
\end{document}

using XeLaTeX. I have Windows 7 and run MiKTeX 2.8.