SITE CHGRP not always working

[bioboy]

SITE CHGRP doesn't always functiona as it should:

lftp ebftpd@localhost:/test> site chgrp test 1
200 User test now has groups: 1                     
lftp ebftpd@localhost:/test> site chgrp test + 2
200 User test now has groups: 1 2                     
lftp ebftpd@localhost:/test> site chgrp test = 3
200 User test now has groups: 3                       
lftp ebftpd@localhost:/test> site chgrp test + 1 2
200 User test now has groups: 3 1 2                     
lftp ebftpd@localhost:/test> site chgrp test - 1
200 User test now has groups: 3