bad request error trying to upgrade to bot account

I created a fresh account (didn't play any games on it) and tried to upgrade it to a bot account (as described here: http://localhost:8000/local.html#tag/Chess-Bot) but am getting a "400 Bad Request" response with the error message "Your browser sent a request that this server could not understand".

Here's the code I used, building on the oauth-personal-token example in the API repository:

const axios = require('axios');

const personalToken = 'my personal token';

axios.get('account/me', {
baseURL: 'https://lichess.org/',
headers: { 'Authorization': 'Bearer ' + personalToken }
}).then(
console.log,
err => console.error(err.message)
);

axios.post('bot/account/upgrade', {
baseURL: 'https://lichess.org/',
headers: { 'Authorization': 'Bearer ' + personalToken }
}).then(
console.log,
err => console.error(err.response.data + " : " + err.message)
);

The GET works perfectly, and the account data is printed (the bot's, not mine, so the token is set up for the right account), including the line

'x-oauth-scopes': 'game:read, preference:read, preference:write, email:read, bot:play',

so setting up the token to include the scope "bot:play" also seems to have worked.

Any idea what could be causing this error?

You have to put an empty object in the second argument of the axios post request.
So change this:
axios.post('bot/account/upgrade', {

into

axios.post('bot/account/upgrade', {}, {

Thanks, @wheatley-o-tron -- that worked!