Blank Report Shows Up For Null Parameters.

I have created a report with a stored proc which takes a parameter whose default value is null. Now when I run my stored proc in Enterprise Manager without any params I get 4 rows otherwise with proper parameter I get 2 rows.

Now I set the params value to allow for Null and blank values. When I run the report if I provide parameter value it works fine but if i try to run it without any param it shows an empty report while the stored proc running without params shows 4 rows.

I have earlier sucessfully created reports where stored procs had 4 params out of which 3 where defaulted to null. And so I set the allow nulls, Allow Blanks values to be true for those params in Rep services. So when I selected value for 1st param and ran the report I got the desired report. But this time I have only one param which can be null. But its not working.

Thanks in advance.