CRM 2016 (Online): use customer in business rule on case

Hi,
In previous versions of Dynamics CRM (Online) we could use the customer field (customerid) on the case in a business rule.

I'm trying to configure it now in a new CRM Online (2016) instance and the customer field doesn't show up in the list of fields. Not in the conditions-section and not in the actions-section. I checked on several options, but I can't seem to find why this can't be used.

Any help is appreciated.

Best regards,

Frank