I bet it must be something simple that is going wrong here.
Firstly, have you checked any other Affiliate apart from ID 18. Are they IDs matching up correctly?
Secondly, have you tried creating a new Affiliate with links. Try to see if you can get the data for that.
Thirdly, is there a chance that you are connecting to the right Infusionsoft Account?
Otherwise, raise a Developer Support Ticket to see if there is something wrong in the account.