OAuth app error: Invalid Authorization Request, No such client

We're creating a integration on our SaaS app that lets multiple customers' Zendesk accounts to authenticate, so we can read their tickets.

1. Created a dev account with Zendesk.

2. Admin → Apps & Integrations → Zendesk API → OAuth Clients → Add client

3. Set Unique identifier as our_test_app

4. Redirect a customer account to https://customer_subdomain.zendesk.com/oauth/authorizations/new?response_type=code&client_id=our_test_app&redirect_uri=http://our_app_domain/api/integrations/zendesk&scope=read

5. Got the Invalid Authorization Request No such client error

Am I missing a step somewhere? Thanks.