Identity Service returns unauthorized

Brand new to Marketo REST API and I'm having trouble calling the identity service.  I've followed the steps in the documentation, created a role for my API user and assigned a user to that role.  Created a new custom service and copied the client_id and client_secret from the view details.  I've copied the identity service URL and added /oauth/token to the end.  I've checked and double-checked the values in my URL and they match what the admin site says.

My GET call URL looks like this:

<myinstanceid>.mktorest.com/identity/oauth/token?client_id=<myclientid>&client_secret=<myclientsecret>&grant_type=client_credentials

No matter what I try, I continually get a 401 error and the json output:

{

  "error": "unauthorized",

  "error_description": "No client with requested id: <myclientid>"

}

(where the <myclientid> is the actual client_id I passed in the GET call, of course).

I can successfully get a token through the UI and I am able to use cUrl to bulk upload a file of attendees, so I think my account is set up correctly.

How can I debug this issue?