XML source doctype

I'm trying to run an XML file through tidy and it seems to choke on the
doctype (the output stops at that line).  If I remove the line, the file is
properly formatted.  Unfortunately, I need the line in there.  I could
remove it, run it through tidy, and then put it back in, but I'd really
prefer not to do that if I don't have to.  I'm hoping I'm missing some
config option or something, but I couldn't find it if I am.  Anyway, here's
what the beginning of the source file looks like:

<?xml version="1.0" ?>
<?fdl version="Beta 1"?>
<!DOCTYPE fenestra-fdl SYSTEM "fenestra-fdl.dtd">

<fenestra-fdl>

  <constraint-package>

[...snip...]


and after formatting:

<?xml version="1.0"?><?fdl version="Beta 1"?>
<!DOCTYPE fenestra-fdl SYSTEM "fenestra-fdl.dtd">

[end of file]


As I said, if I remove the !DOCTYPE tag, it will work fine.  Any ideas?

--Brad