Return value of IsNothing() working on SSRS 2005 and SSRS 2008.

Non-Null: at least one row value in this field is not null.

Null: all row values in this fields are null.

No Rows: this field is empty.

IsNothing	RS2005	RS2008
Non-Null	FALSE	FALSE
Null	TRUE	TRUE
No Rows	TRUE	Null

So, please use the expression count(Fields.myfield.value)=0 to check if the field is empty.