Replace value if its null

I will like to know how I can replace a (A)value in a column for another (B)value if (A) value is null?

I tired this;

=IIF(Fields!actual_date.Value Is Nothing,Fields!program.Value,Fields!program.Value)

What I am trying to do is to check if actual_Date has a value.

If actual_Date has a value then don't display the program value.

If it does not have a value then display the program value.