Seach over empty field not working

Hello,

We have a search on a field that is empty. We want to find all records with a field that are empty (usuario).

It should return 724 rows

but it returns the entire table 2656 rows

Search options:

Version 10.8 Build 39843 x64

Thank you,

We have a search on a field that is empty. We want to find alll records with a field (usuario) that are empty (Imagen36.jpg). It should return 724 rows (imagen38.jpg) but it returns the entire table (2656 rows) (imagen37.jpg)