python sshtunnel的脚本，能连接上去但无法执行命令

脚本如下:

import sshtunnel
from getpass import getpass

ssh_host=YOUR_SSH_HOST
ssh_port=22
ssh_user=YOUR_SSH_USER

REMOTE_HOST=FTP_SERVER
REMOTE_PORT=21

from sshtunnel import SSHTunnelForwarder
ssh_password=getpass('Enter your_SSH_PASSWORD:')

server=SSHTunnelForwarder(
    ssh_address_or_host=(ssh_host,ssh_port),
    ssh_username=ssh_user,
    ssh_password=ssh_password,
    remote_bind_address=(REMOTE_HOST,REMOTE_PORT))

server.start()
print('Connct the remote service via local port: %s' %server.local_bind_port)
#work with ftp service via the server.local_bind_port
try:
    while True:
        pass
except keyboardInterrupt:
    print('Exiting user request.\n')
    server.stop()

 脚本执行起来以后会在本地有一个端口监听，连接这个端口其实就连上了远程的ftp.
但是我执行ls 有报错。
ftp> ls
227 Entering Passive Mode 
ftp: connect: Invalid argument

原因不明