[XeTeX] ** ERROR ** Invalid glyph index (gid 3743)

[Jeremy Van Cleve]

Hello,

I'm finding what might be a conflict between XITS Math and some other 
fonts when I try to use the \underbrace command.  For example,

%%%
\documentclass{article}

\usepackage{unicode-math}
\setmathfont[Scale=MatchLowercase]{XITS Math}
\setmathfont[range=\mathit/{latin,Latin}]{Baskerville Italic}

\begin{document}

\begin{equation}
   \underbrace{xxxxxxxx}
\end{equation}

\end{document}
%%%

gives me "** ERROR ** Invalid glyph index (gid 3743)".  When I comment 
out the XITS Math line, the error goes away.  When I comment out the 
Baskerville line, the error goes away.  Even more strangely, when I 
remove a single "x" from the underbrace, the error also goes away.

Any ideas would be appreciated!

-Jeremy