Twython 403 error: Missing required parameter: Status

Question

I'm building a Markov chain generator using Twython in Python 3.4 that takes my previous tweets as source input, generates a new tweet and posts that to Twitter. The issue I'm having is that when I'm passing the data into the twitter.update_status() method, I get this error:

Exception in Tkinter callback
Traceback (most recent call last):
  File "C:\Python34\lib\tkinter\__init__.py", line 1533, in __call__
    return self.func(*args)
  File "C:\Users\joshs\OneDrive\Documents\markov generator.py", line 80, in send
    twitter.post('statuses/update', params = {'status': tweet})
  File "C:\Python34\lib\site-packages\twython\api.py", line 268, in post
    return self.request(endpoint, 'POST', params=params, version=version)
  File "C:\Python34\lib\site-packages\twython\api.py", line 258, in request
    api_call=url)
  File "C:\Python34\lib\site-packages\twython\api.py", line 194, in _request
    retry_after=response.headers.get('X-Rate-Limit-Reset'))
twython.exceptions.TwythonError: Twitter API returned a 403 (Forbidden), Missing required parameter: status.

I've tried both:

def send():
  global tweet
  twitter.post('statuses/update', params = {'status': tweet})

and:

def send():
  global tweet
  twitter.update_status(status= tweet)

I've even tried explicitly passing the tweet variable as str(tweet) just in case I was being especially stupid and trying to pass a list or something. The string is definitely under 140 characters and doesn't contain any special characters, just A-Z and 0-9.

I'm calling the send() function using a Tkinter button, but I tried without and it still throws this error.

Is there something about not passing variables into Twython functions that I missed in the documentation?

Thanks in advance