Background: I made System field "Assignee" value mandatory on a Transition screen using Script Runner required field Validator.
When I clear the value in assignee field i.e., automatic and make a transition, it shows the error message to fill the field, but it has a -1 value in the field. (First screenshot)
But when I keep unassigned in the assignee field on transition screen and make a transition,it works as expected it shows the error and fills the value of previous assignee in it. 