Custom Summarize Option - What am I doing wrong here?

Hi everyone! I have a table in the database called 'Question Explanations'. There is a column called 'Owner User Id' and this field can either be a numerical number of NULL.

I am trying to create table that gives me the percentage of question explanations where the Owner User Id is null.

Following is my input:

It doesn't work this way and gives me an error :

Am i missing something here? I am a beginner at this so any help will be appreciated. Thankyou!

Did you try using the isnull