Get custom fields using Projects API

Get custom fields using Projects API

Hi! I'm trying to create projects with custom fields via the API. I need the ID's of the custom fields, and I'm using
  1. https://projectsapi.zoho.com/restapi/portals/XXXX/project/customfields/?authtoken=XXXXXXXXX
to get the list of those ID's. However, I keep getting the response " internal exception". Any idea how I can get the list of custom fields?

Thanks!