I am developing a Facebook app for a client. I want to add the client as a tester to the Facebook app so he can play around with it and give me feedback. The client and I are not friends on Facebook, so I am trying to add him by his Facebook ID, as instructed in the Add Tester modal.
When I enter his ID, I get a red x with the error message: "XXXXXXXXXXXX could not be resolved to a valid user ID or name." I have checked that the Facebook ID he gave me is correct via the Facebook Graph API Explorer.
How do I add him using his Facebook ID and get around this problem?