Dynamic link for creating invoice

We have a custom record called Event Registration that we use as an online form to capture customer registrations. One of the fields on this custom record is the actual customer (List/Record) and named as custrecord_er_member.

We're trying to create a hyperlink field on this Event Registration custom record that points to the create invoice page where the customer has been auto-populated using dynamic links. The default value for the hyperlink is set as follows:

https://system.netsuite.com/app/accounting/transactions/custinvc.nl?entity={custrecord_er_member}

Since the custrecord_er_member field points to the customer (as a list/record field), we were expecting the internal id of the customer to be populated in this link; however, what's being populated is the entity id (customer first + last name). Is this the correct behavior or is this a bug?