How to filter IS NULL in ActiveAdmin?

I've a table with an integer column called "map_id", I want to add an activeadmin filter to filter if this column IS NULL or IS NOT NULL.

How could this be implemented ?

I tried the following filter

filter :map_id, :label => 'Assigned', :as => :select, :collection => {:true => nil, :false => ''}

But, I get the following error message :

undefined method `map_eq' for #