[XeTeX] Getting completely the wrong glyphs when using characters from Latin Extended B

[Michiel Kamermans]

I have the following source, in a utf8 encoded text file:

\documentclass{article}
\usepackage{xltxtra}
\setmainfont{FreeSerif}
\begin{document}
Characters from the 01Dx block, all of which are in FreeSerif: ǐ, Ǒ, ǒ, 
Ǔ, ǔ, Ǖ, ǖ, Ǘ, ǘ, Ǚ, ǚ, Ǜ, ǜ, ǝ, Ǟ, ǟ
\end{document}

When I run this through XeLaTeX, using the FreeSerif font, I instead get:

Characters from the 01Dx block, all of which are in FreeSerif: Ȗ, ȗ , Ș, 
ș , Ț, ț , Ȟ, ȟ , Ȧ, ȧ , Ȩ, ȩ , Ȫ, ȫ, Ȭ, ȭ

Where the source has characters from codes 01D0 through 01DF, I can't 
even begin to figure out why the result is what it is: the sequence 
generated is 0216 through 021B, then 021E and 021F, then 0226 through 022B.

What's going on here?

- Mike "Pomax" Kamermans
nihongoresources.com