/login/oauth2/token returns refresh_token not found

[cluemyeein]

I'm attempting to get another access token using my refresh token via the api.  Here's my post data with contentType 

application/x-www-form-urlencoded

POST /login/oauth2/token HTTP/1.1
Host: <my_host>
Content-Type: application/x-www-form-urlencoded

client_id=<my client id>

client_secret=<my client secret>

refresh_token=<my Bearer token>

grant_type=refresh_token
redirect_uri=<my redirect url>

With the above required post data, the response returns: 

302: https://localhost:8443/lti/oauth2response?error=invalid_request&error_description=refresh_token+not+...

The expected response should be something like the one below (which is from the API Doc):

{
    "access_token": "1/fFAGRNJru1FTz70BzhT3Zg",
    "token_type": "Bearer",
    "user": {"id":42, "name": "Jimi Hendrix"},
    "refresh_token": "tIh2YBWGiC0GgGRglT9Ylwv2MnTvy8csfGyfK2PqZmkFYYqYZ0wui4tzI7uBwnN2",
    "expires_in": 3600
  }

I have also tried ContentType application/json

It still returned the aforementioned 302 response.

Can someone help shed some light on what I am doing wrong?

Thanks,
Janice