Missing Values on Table Visualizations

Hi,
I'm doing a simple table, with a Terms aggregations over a field that do not exists in all documents.
I wanted to use the "Show missing values" and assign a default value but the opcion appears disabled
image

Is this opcion available on every field? or there exists restrictions?

Thank you
Regards
Ana

Hi,
I found a solution. I Put {"missing": "1.1.1.1"} in the JSON INput
But, why sometimes appear disabled
Thank you
Regards
Ana