undefined method `type' for nil:NilClass

[amanpoplify]

I am using a custom method in an ActiveAdmin filter, but when I apply the filter, I get the following error:

undefined method 'type' for nil:NilClass

Setup:
ActiveAdmin: 3.2.5
Ransack: 4.3.0
Custom Method: job_title (which retrieves the job title from the Job table)
How can I properly configure this filter without encountering this error?