Problem with Test Runs on Test Execution view issue page when assignee doesn't contain an ID

IllegalStateException: There is no ID mapped for the user key '...' in REST call "/rest/raven/1.0/testexec/.../test"

On earlier versions of JIRA (<6.4), when a Test is assigned to a no longer existing user or user with synchronised but with no JIRA ID, then we get an exception in the Test Issue view.