TidFTP - Error on List method

Hi all
Excuse my ignorance but my knowledge of ftp is miniscule !
I just need my delphi 7 app to download a directory listing from the ftp
server.

When I call the list method of idFTP, I get an exception:
"EidProtocolReplyError with message 'PORT: Not Owner'"

Can anyone tell me what this means.   I'm running the program on Win2k
machine behind a router which does port translation, but I would not have
thought that would cause any issues.

thanks

Mike