How to include NULL in multi value parameters

Thanks BI_Group. But the parameters that i'm using are multi valued and it wont accept NULL as a default value.

Also i did exactly what you mentioned (excluding setting the default value to NULL... I chose to get the values from Query). Here is whats happening. When i chose NULL it gives null records , but when i select any other divname from the drop down list the NULL values are being displayed as well.