how to use where condition which checks for NULL in SSRS?

 Hello,

 I am trying to count data rows in report based on  report field Fields!Coded.Value. I want to count PartID's when Fields!Coded.Value is null and I am not sure how to do it from SSRS expression , I am trying to use below expression  

 in sql I would do it as below ,

   SELECT count(partID)

    From tableName

    where Coded is null

 I am wondering if someone could help me on this?

 Thanks,

 Satya