Ftp-WG: PASV and Snap servers

I'm the author of the Mac FTP client Fetch, and a user has reported 
trouble connecting in PASV mode to a Snap server.  The PASV command 
works, but when Fetch issues any data transfer command and tries to 
make the data connection the response is:

450 Data transfer not implemented for client default data port. Need
'PORT' command first.

Snap has a web page about this problem:

http://www.quantum.com/AM/support/nas/manuals/reference_guide/Compatibility/compatibility_ftp3.html

On that page the mention that some FTP clients are compatible with 
Snap servers in PASV mode, including WS_FTP and Internet Explorer for 
Windows.  My testing suggests that they can add the OS X command-line 
ftp client to that list.  But I've been unable to tell what I'm doing 
differently from the command line client.  Does anyone know the 
secret to using PASV with Snap servers?

Thanks,
-- 
Jim Matthews
Fetch Softworks
http://fetchsoftworks.com