Select User in CustomField

Hello all.

We have a customfield with typ "user".

It may be empty and mutliselect is possible.

In my workflow I want to write the reporter or loggedinuser automatically into the field when it is empty.

rule SystemCustomerReporterRule

when becomesReported() {
  if (customfield.isEmpty) {
  customfield = reporter;
 }
}

The workflow does not work, because the reporter is not a subtype of list<customfield.user>

How can I select the user in the customfield?