Access Rule Expression

In one of the access rules in a controller I have this to control that only admin users can access a specific action:

This works well as long as the user is signed in, but if the user is not signed in and thus do not have any role, I get the error

How can I modify this expression to return false if the role is not defined?

You can add

'users' => array('@'),

before ‘expression’

That’s great, thank you.