I want to be able to make a developer set the assignee to a tester for 'Ready To Test' transition

[SamanthaZoe]

I have a project with 3 different workflows with different rules. I want to change my story and bug workflows so that when a developer transitions a story/bug from 'In Progress' to 'Ready To Test' that they have to change the assignee field. This is because they often forget. 

I already have a validator set that ensures the developer is automatically set as the assignee when they set any story or bug to 'In Progress'. This means the assignee field is already populated. 

I have tried to edit the workflow to set add the following validator:

Field - Assignee

Groups exempt from the validator - none

Error message - "Please select a tester if the ticket is being moved to 'Ready To Test'"

The problem is, when I change the assignee and then press 'Ready To Test' to transition to that state, the error for the validator I have just set is displayed. If I press 'Ready To Test' I do not get a dialog box to change the assignee and transition to 'Ready To Test'.

I have no idea why this is not working.