ftps.storlines socket.timeout despite file upload

问题:

I'm trying to upload a CSV file using ftplib.FTP_TLS, however regardless of the timeout duration I set (5,10,60 seconds), the code always times out with the error:

File "/usr/lib/python3.4/ftplib.py", line 544, in storlines
  conn.unwrap()
File "/usr/lib/python3.4/ssl.py", line 788, in unwrap
  s = self._sslobj.shutdown()
socket.timeout: The read operation timed out

However after the timeout, I check the directory through Cyberduck, and the CSV file is there, complete.

Here is my upload code:

def upload_csv(filename):
    with FTP_TLS(timeout=5) as ftps:
        ftps.set_pasv(True)
        ftps.connect(ftps_server,ftps_port)
        ftps.login(ftps_username, ftps_password)
        ftps.prot_p()
        ftps.cwd('sales')
        ftps.storlines("STOR " + filename, open(filename,'rb'))

I've also tried storbinary(...) but get the same error.

Edit: The file definitely exists, and does actually get created on the server in its entirety. It looks like it's a problem with .shutdown() in ssl.py maybe waiting for a final read, but the internet doesn't seem to yield a solution.

Can anyone shed any light please?