Background: I'm trying to create a filter which identifies the issues where the due date is before today and the last comment author is not the assignee. Basically, I want to find all the past due issues where the assignee hasn't given a reason for being late.
To accomplish this, I first created a custom scripted field called Last Comment Author with this groovy script:

It has a searcher configured:

What I'm trying to then do is run JQL like:

As you can see it wants a searcher, but I have set and reset the searcher and indexed and reindexed JIRA, to no avail.
What am I doing wrong here?