Meaning of fontspec related error

Question

I've edited a document. The edited document works well on my system, but apparently reports this error on another system:

Undefined control sequence.
<argument> ...ontspec_tmp_int\endcsname 255*\real{0.5}\relax

What is this error about?

The error appears when calling \maketitle which has been redefined to (not my doing):

\def\maketitle{
  \thispagestyle{empty}
    \vfill
  \begin{raggedright}
  \leavevmode
    \vskip 1cm
    {\fontsize{50}{60}\fontspec[Color=red,Opacity=0.5]{Adobe Garamond Pro}\textbf{\@title}\par}
    \vskip 1cm
    \normalfont
    {\Huge {\textit{\@author}\par}}
  \vfill
        {\Large Name}
    \newline
          {\Large \href{mailto:name@name.edu}{name@name.edu}}
        \newline
          {\Large \href{http://name.com}{http://name.com}}
        \newline
    {\Large \@date\par}
   \end{raggedright}
  \null
  \cleardoublepage
}

Answer 1

This error is due to changes in the LaTeX3 code that fontspec depends on. This code used to load the calc package, but no longer does, and fontspec hasn't yet reflected this change. The offending line of code is the \fontspec command, which sets the font opacity. You can solve the problem by adding.

\usepackage{calc}

to your preamble. It is likely that the this will be fixed relatively soon.

By the way, the offending line is not the way you should do this. In general, you should avoid using the \fontspec command to change fonts. Instead, you should define a new font family outside your title definition using e.g.

\newfontfamily\mytitlefont[Color=red,Opacity=0.5]{Adobe Garamond Pro}

and then inside your title redefinition use \mytitlefont.