mathdesign compiles ß to SS

Question

I'd like to use the package mathdesign, but it somehow makes problems with the german letter ß. My question is related to XeTeX: “ß” compiles as “SS”, German umlauts work but different because the reason seems to be mathdesign here.
The effect is best show with the following minimal example

\documentclass{standalone}
\usepackage{polyglossia}
\setdefaultlanguage[spelling=new,babelshorthands=true]{german}%
%\usepackage{utopia}\usepackage[utopia]{mathdesign}
\begin{document}
The german ß compiles to SS when used with \emph{mathdesign} package.
\end{document}

beeing compiled with line 4 uncommented resulting in or with line 4 activated resulting in .
Ä, Ö, Ü and so on works fine.

Answer 1

Unless you really have to use polyglossia, you can try the following, which seems to work fine on my machine:

\documentclass{scrartcl}
\usepackage[ngerman]{babel}
\usepackage[utopia]{mathdesign}
\usepackage{fontspec}
\setmainfont{Utopia}
\begin{document}
The german ß compiles to ß when used with \emph{mathdesign} package.
\begin{equation}
  a^2 = b^2 + c^2
\end{equation}
\end{document} 

This uses the OpenType version of Utopia (for text), available here, which I think is a bit more appropriate in XeTeX/LuaTeX. For math, mathdesign remains in use.