How to get only Messages where some specific custom field have value?

I tried this out and unfortunately it doesn't seem to work. Our community has a country custom field.

Here I used c_country!="" as a constraint and got back users with no value for c_country.

Akenefick_0-1689338558934.png

Then I tried using a specific country as a constraint.

Akenefick_1-1689338775358.png

It seems like this should work but apparently you can't use custom fields as LiQL constraints.