Troubleshooting SDK.REST.RetrieveRecord

A couple years ago I posted here about how to use this method: https://community.dynamics.com/crm/f/117/t/242500

I solved the issue as noted there, but now I'm trying the same thing an another entity and it's not working.

The code is on the new_Invoice entity, and I'm trying to access a field on the out-of-box Account entity.

Here's my code:

var companyID = Xrm.Page.getAttribute('new_companyid').getValue()[0].id;
var APContact = null;
var name = null;

SDK.REST.retrieveRecord
(
  companyID,
  'Account',
  null, null,
  function (Account)
  {
    APContact = Account.new_AccountsPayableContactId;
  },

  function Error()
  {
    alert('There is an error in reading account data.');
  }
);

In all my tests, APContact has always ended up null within the function and after it. What am I doing wrong?