Fillnull command is not working in my search for specific sourcetype

Hi, I been using fill null commands on my other searched without any issue, but in a specific case i am unable to get any response by using fillnull, the data is indexed by a source type called CSV, (specific for CSV files), I will have 1000's of empty values in fields so I need to filter our based on my needs. one on my need is to filter it my means if null values. SO I want to replace the empty values in a filled with value-NULL. I used below format. field name =""RWI State" and i used the fillnull as ....| fillnull value=NULL "RWI State". but its not filling the filed with NULL values Thanks