[Xy-pic] Math fonts

[Yannick Delbecque]

After a few tests, I get the same error with the minimal file:

\documentclass{article}
\usepackage[all]{xy}
\CompileMatrices
\begin{document}
\[ \xymatrix{ \mathrm{R} } \]
\end{document}

But there is no error if I desactivate "\CompileMatrices" or remove the 
\mathrm command!

I also checked when there is no .xyc file present, and the error still 
occurs: the problem doesn't seems to be the reading of the compiled 
file by xypic.

Note I used both OzTeX and TeTeX under mac os X to verify this.

Thanks!
Yannick