Reports - Criteria with is null not working

I have a form with an approval date field (Type Date) and an approved by field (Type text) which is populated using a stateless approval form through a custom action. To create a pending for approval report that lists only the records where approval date is null, I have created a report and set the criteria where approval date is null. The report shows no records even though records exist.

Alternately I tried using the approved by field and set the criteria to where approved_by = "" (the option of is null does not show up for this field). Here again the report shows no records. However for both methods when the criteria is set to a value that exists it shows the records correctly.

Is there an issue with reports when criteria is set to null or "" ?

Thanks