Paramiko SSH "put", destination name

[j.crater]

Hello all, I am having difficulties understanding paramiko's behaviour, when it comes to sending files over SSH. I have this function:

def SSH_send_file(file, host, username, password, path):   
   s = paramiko.SSHClient()
   s.set_missing_host_key_policy(paramiko.AutoAddPolicy())
   s.connect(host, 22, username, password)
   sftp = s.open_sftp()
   sftp.put(file, path)
   s.close()

For example, you would call it like:
SSH_send_file("file_to_send.txt", "10.20.30.40", "user", "secret", "/tmp")

Until now, it worked fine using it like in the example above. But now (in same program) there was an occassion when this error happened:

Error:
  File "\lib\site-packages\paramiko\sftp_client.py", line 811, in _convert_status
    raise IOError(text)
OSError: Failure

After some Googling I found that solution is to pass complete destination name to function, which means the file name included, like this:
SSH_send_file("file_to_send.txt", "10.20.30.40", "user", "secret", "/tmp/file_to_send.txt")

Why did this function "change behaviour", so I had to use it differently in one case, as opposed to previous uses? Can I avoid/predict it in future?
Thank you! JC