Optional Prompt value displayed in report

Hi,

I have the formula in the report which doesn't seem to work.

=If (Not (IsNull( UserResponse("Department Name:")))) Then  "ALL Departments"   Else UserResponse("Department Name:")

This is an optional prompt. When the user selects a Dept name, it should display the selection, if not Display "ALL Department"

Any help is appreciated.

Thank You

Veena M