Breiter Gravis für {xe|lua}latex + unicode-math

\documentclass{article}
\pagestyle{empty}
\usepackage{unicode-math}
%%% siehe http://texwelt.de/fragen/26664/breiter-gravis-fur-pdflatex/26908
\usepackage{graphicx}
\makeatletter
\newcommand\widegrave[1]{%
  \mathpalette\@wideaccent{{#1}{\mathchar"7012}}%
}
\newcommand*\@wideaccent[2]{%
  \@@wideaccent{#1}#2%
}
\newcommand\@@wideaccent[3]{%
  \setbox\@tempboxa\hbox{$\m@th#1#2$}%
  \@tempdima=\ht\@tempboxa
  \@tempdimb=\wd\@tempboxa
  \ooalign{%
    \box\@tempboxa\cr
    \hidewidth\raise\dimexpr\@tempdima-1ex\relax\hbox{\resizebox{\@tempdimb}{\height}{$\m@th#1#3$}}\hidewidth
  }%
}
\makeatother
\begin{document}
Gut: \(\grave{a}\) \ldots\ \(\grave{z}\) \(\grave{A}\) \ldots\ \(\grave{Z}\) \(\grave{\alpha}\) \ldots\ \(\grave{\omega}\) \(\grave{\Gamma}\) \ldots\ \(\grave{\Omega}\) \(\grave{\aleph}\)\\
Beschissen: \(\grave{lang}\)\\
Kompiliert nicht: \(\widegrave{x}\ \widegrave{lang}\)
\end{document}

! Package graphics Error: Division by 0.

See the graphics package documentation for explanation.
Type  H <return>  for immediate help.
 ...

l.26 Kompiliert nicht: \(\widegrave{x}
                                      \ \widegrave{lang}\)
? Q