Error (?) uploading

While uploading files to a Synology nas, with a bash script that call a 
function like this

lftp -f "
open $HOST
user $USER $PASS
lcd $SOURCEFOLDER
mirror --reverse --delete --verbose --dereference --use-cache 
--only-newer $SOURCEFOLDER $TARGETFOLDER
bye
"

I got this error many and many times: lftp is the Debian package 4.7.7

chmod: Operazione non supportata: MFF and SITE CHMOD are not supported 
by this site

But uploads are done. I'm not so expert about ftp, is it really a 
problem or a simple warning?
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