[XeTeX] Prime + Unicode-math

[José Carlos Santos]

Hi all,

If I compile this file:

\documentclass{article}
\usepackage{fontspec}
\usepackage{xltxtra}
\usepackage{unicode-math}
\begin{document}
$P'$
\end{document}

I get this error message:

! Undefined control sequence.
<argument> \um_prime_single_mchar

l.6 $P'$

Why is that? The unicode-math documentation says:

In regular LATEX, primes can be entered with the straight quote 
character ', and multiple straight quotes chain together to produce 
multiple primes. [...] For Unicode maths, we wish to conserve this 
behaviour.

Therefore, I don't see why this happens.

If, instead of the straight quote character ' I use a prime ′, then I 
get no error message, but I get:

Missing character: There is no ′ in font cmr10!

What should I do in order to get the prime?

Best regards,

José Carlos Santos