I cannot get past step 4 of the quickstart: http://nationbuilder.com/api_quickstart

I've made sure that my client id, secret, redirect url and code are all accurate and up to date. 

Request:

curl -X POST --header "Content-Type: application/json" --header "Accept: application/json" --data '{"grant_type":"authorization_code", "client_id":"CLIENTID", "client_secret":"SECRET", "redirect_uri":"http://localhost:9000", "code":"CODE"}'  https://DOMAIN.nationbuilder.com/oauth/token

Note: things in capitals have been redacted

Response:

{"error":"invalid_grant","error_description":"The provided authorization grant is invalid, expired, revoked, does not match the redirection URI used in the authorization request, or was issued to another client."}

Any thoughts as to what could be going on?