It shows -1 in assignee field on a transition screen

[Arunajyothi Samala]

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.  

[Uday Kiran Bhaviri]

Hi @Arunajyothi Samala  this shows that it didn't really clear the assignee value. It still has that reference and a value in it.

[Arunajyothi Samala]

in the UI it has a value meaning it has value in the VIEW screen. I am forcing/erasing it on a transition screen,so when I clear it and make a transition it happens a -1