Expression JQL function doesn't work with user-picker custom fields

If you try to make comparisons involving a user-picker custom field (including script fields) in the JQL Expression function, you will be met with the following error:

Field LastCommentAuthor found but has no searcher. Edit it, set the searcher, and reindex.

Where "Last Comment Author" is an example custom user-picker field.

Steps to reproduce:

• Create either a new custom user-picker field or a Script field that returns an ApplicationUser.
  • Set the field to show on the appropriate screens
  • Ensure that your field has the proper searcher and template
• Create an issue and give your new field a user value.
• Now search your issues and try to execute a JQL user comparison using the Expression function like the following:

  issueFunction in expression("","CustomFieldName != Assignee")

• Upon executing your JQL, observe the error that was mentioned above.