[plt-scheme] bytes-open-converter question

Okay, I don't want to report this as a bug before sending it to the 
list, because I expect it's something stupid I'm doing.
First of all, the basics:
Platform: Windows XP
PLT-Scheme version 352, using DrScheme with Swindle.

I am trying to convert documents from any of a bunch of different 
encodings to UTF-8 to use as training data. I installed the whole 
shebang from plt-scheme.org and things work fine from the default 
encoding to UTF-8, but I can't convert from any other encodings, 
including ASCII.  I checked to see that iconv.dll was in my lib 
directory and in the same directory as libmzschVERSION.dll, it was.  In 
fact, here's the contents of the directory, which has not been modified 
since installation.

UnicoWS.dll  libeay32.dll        libmzsch352_000.dll  myssink.dll
gcc          libmred352_000.dll  msvc                 ssleay32.dll
iconv.dll    libmzgc352_000.dll  myspage.dll

Here are some examples of what does/doesn't work:

Welcome to DrScheme, version 352.
Language: CustomSwindle.
 > (bytes-open-converter "" "UTF-8")
WARNING: The definitions window has changed. Click Run.
#<string-converter>
 > (bytes-open-converter "WINDOWS-1251" "UTF-8")
WARNING: The definitions window has changed. Click Run.
#f
 > (bytes-open-converter "CP1251" "UTF-8")
WARNING: The definitions window has changed. Click Run.
#f
 > (bytes-open-converter "ASCII" "UTF-8")
WARNING: The definitions window has changed. Click Run.
#f
 >


Any ideas?  Thanks!

-- Jeff