isset in ASP...type mismatch ouch

Hi all is there a way to check whether a variable is empty.

Code below "no funciona" as they say in Spain.

if (IsEmpty(Request.QueryString("year")) Then
                year =  Year(now)
                Else
                year = Request.QueryString("year")
                End if

Code (markup):

 

IsNull

If len < "a number"