Report parameter not getting updated based on the another report parameter (multi value) when deselecting all values (emptying the selections)

Consider the below example:
Dept Student
CSE  A1
CSE  A2
CSE  A3
ECE   B1
ECE   B2
ECE   B3
EEE   C1
EEE   C2
EEE   C3
I have 2 parameters for a report, the first report param is Dept (CSE, ECE, EEE) with multi selection. 2nd report parameter is Student name with multi selection. By default all the Dept selected and the student names listed. when changing the selection the 2nd parameter gets updated properly, but its not updating when deselecting all Dept values.
For eg, when having selected CSE Dept, i have Student list with A1, A2 and A3 values. if i deselect the CSE there are 2 cases:
1)In Visual studio preview, its not allowing me to deselect CSE,
2)In Report manager site, its allowing me to deselect all but even after deselecting i can still see A1,A2 and A3 in the student list.
I have checked in the SQL profiler when deselecting all the values the script/sp to update the 2nd param is not triggered.
I want to empty the 2nd param when no value selected in 1st param.

Could anyone please help me on this.

Thanks and Regards