'Unauthorized client: `grant_type` is invalid'

[namjungsoo]

Hello,
I got that error from MoneyButtonClient when I login with email and password in Node.js.
Is there any special OAuth app certification process in Apps at moneybutton.com?
I cannot find any grant_type setting method in the Apps.

Here are codes:

var moneybutton = require('@moneybutton/api-client')
var constants = require('./constants')

async function main() {
  var mbclient = new moneybutton.MoneyButtonClient(constants.CLIENT_ID, constants.CLIENT_SECRET)
  await mbclient.logIn('namjungsoo@gmail.com', 'MyPassword')
  console.log(await mbclient.whoAmI())
}

main()

and the error message:

node mbclient.js 
[
  {
    id: 'b800b6c0-5f48-11ea-be9a-496277f34fa6',
    status: 400,
    title: 'Bad Request',
    detail: 'Unauthorized client: `grant_type` is invalid'
  }
]

Thank you