Sending POST request in AJAX with Python3.4 and urllib

I am trying to clear line movements from: http://scores.covers.com/football-scores-matchups.aspx

I need to iterate over every season and week of using the Calendar:

When I check the network to see what is being sent, I see two POST requests when I change the Season:

(I merged the above 3 images into 1 as I don't have enough posts to post more than 1 image)

http://s29.postimg.org/773muclra/covers_scrape4.jpg

I've searched, read and hacked it all day and made no progress. I can recreate the error I get using Firebug to edit and resubmit the post request if I change the payload to be something unexpected. So I feel like the problem is with how I am coding and posting data. I've tried json.dump, utf-8, Content-Type application / x-www ... in every combination I can think of.

My current code looks like this:

import urllib.request
import json
import urllib.parse
class Scraper():

    def __init__(self):
        pass

    def scrape(self):
        url = 'http://scores.covers.com/ajax/SportsDirect.Controls.LiveScoresControls.ScoresCalendar,SportsDirect.Controls.LiveScoresControls.ashx?_method=changeDay&_session=no'

        data = {
            'league': '1',
            'SeasonString': '2012-2013',
            'Year': '1',
            'Month': '1',
            'Day': '1'
        }        

        # data = urllib.parse.urlencode(data).encode('utf-8')
        data = json.dumps(data).encode('utf-8')

        headers = {
            'Host': 'scores.covers.com',
            'User-Agent': 'Mozilla/5.0 (Windows NT 6.3; WOW64; rv:32.0) Gecko/20100101 Firefox/32.0',
            'Accept': 'text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8',
            'Accept-Language': 'en-US,en;q=0.5',
            'Accept-Encoding': 'gzip, deflate',
            'Referer': 'http://scores.covers.com/football-scores-matchups.aspx',
            'Content-Type': 'application/json',
            'Connection': 'keep-alive',
            'Pragma': 'no-cache',
            'Cache-Control': 'no-cache'
        }

        req = urllib.request.Request(url)
        req.data = data
        req.headers = headers

        f = urllib.request.urlopen(req)
        print(f.read())

        return f

      

        
        
        
      

    

What gives:

In [1]: import scraper

In [2]: s = scraper.Scraper()

In [3]: s.scrape()
b"new Object();r.error = new ajax_error('System.ArgumentException','Object of type \\'System.DBNull\\' cannot be convert
ed to type \\'System.String\\'.',0)"
Out[3]: <http.client.HTTPResponse at 0x4d6b650>

      

        
        
        
      

    

Thanks in advance.