Parameter not working

Hi All,

I'm using Report Builder 3.0

I have a report running against an Oracle DB. I have a sql in the main data set. In the main dataset i added a parameter at the end "WHERE BKNGTYP IN (:BKNGTYP)". I created a new dataset for parameter (select distinct bkntyp from mainquery) and i have the following in the result set (OUT,WAR,REM,NULL)

But when i run the report and select NULL , no records are displayed in the report, where as there are records for the NULL Selection. I tested this using TOAD AND SQL DEVELOPER. I get records when i select any other values like OUT, WAR or REM.

Please need help. 

Thanks