requests.post only accepts "text" dictionary values

Question

I'm attempting to make post requests to slack from a requests.post() call. Here is an example of the code that I've made to do this:

    fields = {
        'sender': message['From'],
        'subject': message['Subject'],
    }

    url = 'https://hooks.slack.com/services/XXXXXX'
    print(fields)
    return requests.post(url, json=fields)

The issue seems to be that the requests.post(url, json=fields) command only seems to respond to the fields command if I change the key value(s) to "text". I have no idea what is not allowing the requests.post command to use other key values (such as sender or subject) that I have defined here.

I have been able to fix this by doing a terrible workaround of calling two separate requests.post commands and setting sender/subject as their own variables with json formatted dictionaries:

sender = {'text': message['From']}
subject = {'text': message['Subject']}

Just a side-note I'm gathering these message handlers from aiosmtpd.handlers which allows me to parse smtp messages that come in.